Add experimental C/C++ implementation for process context OTEP

[ivoanjo]

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):

$ ./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.