This PR adds an experimental C/C++ implementation for the "Process
Context" OTEP being proposed in
open-telemetry/opentelemetry-specification#4719

This implementation previously lived in
https://github.com/ivoanjo/proc-level-demo/tree/main/anonmapping-clib
and as discussed during the OTEL profiling SIG meeting we want to add
it to this repository so it becomes easier to find and contribute to.

I've made sure to include a README explaining how to use it. Here's
the ultra-quick start (Linux-only):

```bash
$ ./build.sh
$ ./build/example_ctx --keep-running
Published: service=my-service, instance=123d8444-2c7e-46e3-89f6-6217880f7123, env=prod, version=4.5.6, sdk=example_ctx.c/c/1.2.3, resources=resource.key1=resource.value1,resource.key2=resource.value2
Continuing forever, to exit press ctrl+c...
TIP: You can now `sudo ./otel_process_ctx_dump.sh 267023` to see the context

 # In another shell
$ sudo ./otel_process_ctx_dump.sh 267023 # Update this to match the PID from above
Found OTEL context for PID 267023
Start address: 756f28ce1000
00000000  4f 54 45 4c 5f 43 54 58  02 00 00 00 0b 68 55 47  |OTEL_CTX.....hUG|
00000010  70 24 7d 18 50 01 00 00  a0 82 6d 7e 6a 5f 00 00  |p$}.P.....m~j_..|
00000020
Parsed struct:
  otel_process_ctx_signature       : "OTEL_CTX"
  otel_process_ctx_version         : 2
  otel_process_ctx_published_at_ns : 1764606693650819083 (2025-12-01 16:31:33 GMT)
  otel_process_payload_size        : 336
  otel_process_payload             : 0x00005f6a7e6d82a0
Payload dump (336 bytes):
00000000  0a 25 0a 1b 64 65 70 6c  6f 79 6d 65 6e 74 2e 65  |.%..deployment.e|
00000010  6e 76 69 72 6f 6e 6d 65  6e 74 2e 6e 61 6d 65 12  |nvironment.name.|
...
Protobuf decode:
attributes {
  key: "deployment.environment.name"
  value {
    string_value: "prod"
  }
}
attributes {
  key: "service.instance.id"
  value {
    string_value: "123d8444-2c7e-46e3-89f6-6217880f7123"
  }
}
attributes {
  key: "service.name"
  value {
    string_value: "my-service"
  }
}
...
```

Note that because the upstream OTEP is still under discussion, this
implementation is experimental and may need changes to match up with
the final version of the OTEP.

**What does this PR do?**

This PR bumps the libdatadog dependency from version 28.0.2.1.0 to
29.0.0.1.0.

This new version brings:

* macOS build fixes needed to unblock #5351
* libdatadog support for the
  [OTel process context](open-telemetry/opentelemetry-specification#4719)
  (I plan to submit a PR with some integration for testing this
  separately -- commit is already waiting)

**Motivation:**

Adopt latest libdatadog.

**Change log entry**

Yes. Upgrade libdatadog dependency to version 29.0.0

**Additional Notes:**

N/A

**How to test the change?**

Green CI is good, as usual.

**What does this PR do?**

This PR adds integration tests for the
[libdatadog](DataDog/libdatadog#1658) support
for
[OTel process context](open-telemetry/opentelemetry-specification#4719).

**Motivation:**

Starting in libdatadog v29, the process context is automatically
published together with the process discovery feature, so we don't
need to do anything from the Ruby side to turn it on.

Yet, we want to make sure this is in good shape as this is a WIP
effort so having a few Ruby-level integration tests seems nice.

**Additional Notes:**

For Datadog folks -- more details on this work can be found on
 #ebpf-context-propagation on our slack.

The process context is stored as a protobuf message in a
very special memory location, hence the new test using
protobuf + the weird code to parse memory.

**How to test the change?**

Validate that new spec is passing!

### What does this PR do?

Update OTel process context to v2 (see open-telemetry/opentelemetry-specification#4719)

### Motivation

Bring implementation in dd-trace-go up-to-date with proposal.

### Reviewer's Checklist
<!--
* Authors can use this list as a reference to ensure that there are no problems
  during the review but the signing off is to be done by the reviewer(s).
-->

- [x] Changed code has unit tests for its functionality at or near 100% coverage.
- [ ] [System-Tests](https://github.com/DataDog/system-tests/) covering this feature have been added and enabled with the va.b.c-dev version tag.
- [ ] There is a benchmark for any new code, or changes to existing code.
- [ ] If this interacts with the agent in a new way, a system test has been added.
- [ ] New code is free of linting errors. You can check this by running `make lint` locally.
- [ ] New code doesn't break existing tests. You can check this by running `make test` locally.
- [ ] Add an appropriate team label so this PR gets put in the right place for the release notes.
- [ ] All generated files are up to date. You can check this by running `make generate` locally.
- [x] Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running `make fix-modules` locally.

Unsure? Have a question? Request a review!

JIRA [PROF-14086] 

Co-authored-by: nicolas.savoire <[email protected]>