…the DSL (#12319)

This PR removes the DSL from the code generator, in anticipation of splitting the DSL out into a separate package.

Given a .proto file like:

```proto
syntax = "proto3";

package pkg;

message TestMessage {
  optional int32 i32 = 1;
  optional TestMessage msg = 2;
}
```

Generated code before:

```ruby
# Generated by the protocol buffer compiler.  DO NOT EDIT!
# source: protoc_explorer/main.proto

require 'google/protobuf'

Google::Protobuf::DescriptorPool.generated_pool.build do
  add_file("test.proto", :syntax => :proto3) do
    add_message "pkg.TestMessage" do
      proto3_optional :i32, :int32, 1
      proto3_optional :msg, :message, 2, "pkg.TestMessage"
    end
  end
end

module Pkg
  TestMessage = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("pkg.TestMessage").msgclass
end
```

Generated code after:

```ruby
# frozen_string_literal: true
# Generated by the protocol buffer compiler.  DO NOT EDIT!
# source: test.proto

require 'google/protobuf'

descriptor_data = "\n\ntest.proto\x12\x03pkg\"S\n\x0bTestMessage\x12\x10\n\x03i32\x18\x01 \x01(\x05H\x00\x88\x01\x01\x12\"\n\x03msg\x18\x02 \x01(\x0b\x32\x10.pkg.TestMessageH\x01\x88\x01\x01\x42\x06\n\x04_i32B\x06\n\x04_msgb\x06proto3"
begin
  Google::Protobuf::DescriptorPool.generated_pool.add_serialized_file(descriptor_data)
rescue TypeError => e
  # <compatibility code, see below>
end

module Pkg
  TestMessage = ::Google::Protobuf::DescriptorPool.generated_pool.lookup("pkg.TestMessage").msgclass
end
```

This change fixes nearly all remaining conformance problems that existed previously. This is a side effect of moving from the DSL (which is lossy) to a serialized descriptor (which preserves all information).

## Backward Compatibility

This change should be 100% compatible with Ruby Protobuf >= 3.18.0, released in Sept 2021. Additionally, it should be compatible with all existing users and deployments. However there is some special compatibility code I inserted to achieve this level of backward compatibility.

Without the compatibility code, there is an edge case that could break backward compatibility. The existing code is lax in a way that the new code would be more strict.

When we use a full serialized descriptor, it will contain a list of all `.proto` files imported by this file (whereas the DSL never added dependencies properly): https://github.com/protocolbuffers/protobuf/blob/dfb71558a2226718dc3bcf5df27cbc11c1f72382/src/google/protobuf/descriptor.proto#L65-L66

`add_serialized_file` will verify that all dependencies listed in the descriptor were previously added with `add_serialized_file`.  Generally that should be fine, because the generated code will contain Ruby `require` statements for all dependencies, and the descriptor will fail to load anyway if the types we depend on were not previously defined in the DescriptorPool.

But there is a potential for problems if there are ambiguities around file paths. For example, consider the following scenario:

```proto
// foo/bar.proto

syntax = "proto2";

message Bar {}
```

```proto
// foo/baz.proto

syntax = "proto2";

import "bar.proto";

message Baz {
  optional Bar bar = 1;
}
```

If you invoke `protoc` like so, it will work correctly:

```
$ protoc --ruby_out=. -Ifoo foo/bar.proto foo/baz.proto
$ RUBYLIB=. ruby baz_pb.rb
```

However if you invoke `protoc` like so, and didn't have any compatibility code, it would fail to load:

```
$ protoc --ruby_out=. -I. -Ifoo foo/baz.proto
$ protoc --ruby_out=. -I. -Ifoo foo/bar.proto
$ RUBYLIB=foo ruby foo/baz_pb.rb
foo/baz_pb.rb:10:in `add_serialized_file': Unable to build file to DescriptorPool: Depends on file 'bar.proto', but it has not been loaded (Google::Protobuf::TypeError)
	from foo/baz_pb.rb:10:in `<main>'
```

The problem is that `bar.proto` is being referred to by two different canonical names: `bar.proto` and `foo/bar.proto`. This is a user error: each import should always be referred to by a consistent full path. Hopefully user errors of this sort are rare, but it is hard to know without trying.

The code in this PR prints a warning using `warn` if we detect that this edge case has occurred. We will plan to remove this compatibility code in the next major version.

Closes #12319

COPYBARA_INTEGRATE_REVIEW=#12319 from haberman:ruby-gencode-binary 5c0e8f2
FUTURE_COPYBARA_INTEGRATE_REVIEW=#12319 from haberman:ruby-gencode-binary 5c0e8f2
PiperOrigin-RevId: 524059189