Continuous profiler - Splunk exporter

[Kielek]

Relays on changes from open-telemetry/opentelemetry-dotnet-instrumentation#3196

It is a port of profiler-exporter from https://github.com/signalfx/signalfx-dotnet-tracing
It brings Vendored version of protobuf-net.

The test is focus on testing the exporter. The frames detected by the native code (Vb, F#, C++) are tested in the upstream.

Tested with local build (both, upstream and the Splunk plugin). For now, there is no possibility to execute tests in CI.

Follow ups:

1. Verify if we can drop protobuf-net and relay only on the same libraries as OpenTelemetry.Exporter.OpenTelemetryProtocol.
2. Add test for allocation sampling (it is not available in the SFx version).
3. Update upstream to work it in CI/in release.

Notes

While merging, please add following comment:

Co-authored-by: John Bley <[email protected]>
Co-authored-by: Paulo Janotti <[email protected]>
Co-authored-by: Robert Pająk <[email protected]>
Co-authored-by: Mateusz Łach <[email protected]>
Co-authored-by: Dawid Szmigielski <[email protected]>