…C` helper

**What does this PR do?**

This PR introduces a new `DDOG_CHARSLICE_C_BARE` helper, as well
as undoes the change to `DDOG_CHARSLICE_C` from
https://github.com/DataDog/libdatadog/pull/305/files#diff-5c00b9c49c81bf0d0a754e0bcc6e0add4fe849bf0ea4a4d1ebfac8f698150f0b

There are now two helpers: `DDOG_CHARSLICE_C` that includes the
`(ddog_CharSlice)` cast and `DDOG_CHARSLICE_C_BARE` which doesn't.

**Motivation:**

In a nutshell @bwoebi explained to me that on Windows there's times
where you need to use `(ddog_CharSlice) {.ptr = ..., .len = ...}` and
others where you need to use `{.ptr = ..., .len = ...}` and cannot
include the cast.

Thus in #305 we tweaked the helper to never have `(ddog_CharSlice)`,
with the intention of using `(ddog_CharSlice) DDOG_CHARSLICE_C(...)`
some times, and `DDOG_CHARSLICE_C(...)` the other times.

But... this had the downside of making some of our Ruby Profiler code
really ugly, as we'd need to move from `DDOG_CHARSLICE_C` to
`(CharSlice) DDOG_CHARSLICE_C` in a lot of cases.

Thus, this PR introduces one helper for each variant, so that
we always have a nice user-friendly option for each case, and avoid
needing to sprinkle casts.

**Additional Notes:**

Arghhh C/C++ you always manage to be weird and annoying.

**How to test the change?**

I've tested this change by compiling the Ruby profiler to use this
branch of libdatadog, and validating that it compiles successfully
on Linux.

CI was failing on windows with:

```
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(35,16): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(36,15): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(55,23): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(56,27): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(62,14): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(63,14): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(79,36): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(102,31): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(106,32): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(114,30): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(114,68): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(115,30): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(127,15): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(138,1): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
D:\a\libdatadog\libdatadog\examples\ffi\exporter.cpp(141,1): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
```