fix: Allow overlapping context scopes

[bantonsson]

Fixes #1887

This PR is fully functional, but not complete with edge case checks, tests and comments. It's ment as a basis for discussions.

Changes

This PR changes the single current Context to a more robust stack that is resilient to out of order and overlapping Context scopes. This robustness is needed to be able to support better interop with tokio-rs/tracing.

There are some context_attach benchmarks added, and this implementation performs the same or between 3-17% faster than the old one (while maintaining a working context state).

As for the context benchmarks in opentelemetry-sdk, this implementation performs the same except for the context/is_sampled/?/alt variants where it is 6-7% slower. I haven't dug into this too much at this point.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 96.20253% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.9%. Comparing base (68af3bb) to head (ab2f259).

Files with missing lines	Patch %	Lines
opentelemetry/src/context.rs	96.2%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2378   +/-   ##
=====================================
  Coverage   77.9%   77.9%           
=====================================
  Files        123     123           
  Lines      22944   23016   +72     
=====================================
+ Hits       17880   17949   +69     
- Misses      5064    5067    +3     

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