Add Linux user_events receiver

[lalitb]

Change Summary

Split out from the original combined PR #2717.

Adds a contrib receiver for Linux user_events tracepoints.

The receiver supports two decode formats:

• tracefs: generic Linux tracepoint decoding based on the static schema exposed in tracefs format files.
• event_header: decodes self-describing EventHeader payloads. EventHeader comes from the Microsoft LinuxTracepoints-Rust project, but is open and usable by any producer.

The receiver uses one_collect for perf/user_events collection, and supports single or multiple subscriptions, batching, late registration, and basic receiver metrics. It is behind the userevents-receiver feature flag.

Also adds a Linux CI smoke test that registers and reads both tracefs and EventHeader user_events tracepoints when kernel support is available.

What issue does this PR close?

• Closes Proposal: Add Linux user_events Receiver for Telemetry from Applications Co-Located with the Collector #2698

How are these changes tested?

Added Linux CI smoke test

Are there any user-facing changes?

Yes, a receiver.

[codecov]

Codecov Report

❌ Patch coverage is 61.06195% with 704 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.91%. Comparing base (524b78d) to head (7f26d8c).

❌ Your patch check has failed because the patch coverage (61.06%) is below the target coverage (70.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2787      +/-   ##
==========================================
- Coverage   86.09%   85.91%   -0.18%     
==========================================
  Files         694      699       +5     
  Lines      263421   265229    +1808     
==========================================
+ Hits       226789   227877    +1088     
- Misses      36108    36828     +720     
  Partials      524      524              

Components	Coverage Δ
otap-dataflow	86.82% <61.06%> (-0.23%)	⬇️
query_abstraction	80.61% <ø> (ø)
query_engine	90.76% <ø> (ø)
otel-arrow-go	52.45% <ø> (ø)
quiver	92.25% <ø> (ø)

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