Add profiler auto instrumentation

[russcam]

This commit adds profiler auto instrumentation to the
Elastic APM .NET agent solution.

Profiler auto instrumentation uses the CLR profiling APIs
to instrument methods of interest at runtime, by signalling
to the runtime to load an unmanaged profiler into the process
using profiling environment variables.

The approach comprises:

1. A CLR profiler written in Rust
2. Supporting managed assemblies containing methods
   that will be inserted into rewritten IL and called, to start
   and end transactions and spans.

Initial integrations are available for common ADO.NET providers
and Kafka, with more to follow in the future.

The profiler auto instrumentation approach is currently
an alpha release, as indicated by the version number
in src/elastic_apm_profiler/Cargo.toml.

Closes #1522

[apmmachine]

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• Start Time: 2021-11-03T06:52:25.251+0000

• Duration: 85 min 19 sec

• Commit: 477899c

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Skipped	7
Total	8365

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