chore: Move FutureExt into context

[scottgerring]

Fixes #2754

Changes

• Switch opentelemetry::context from context.rs to context/mod.rs module style so that we can add future_ext without jamming everything together into the same file
• Move FutureExt from opentelemetry::trace::context into opentelemetry::context::future_ext
• Re-export FutureExt in trace to maintain backwards compatibility

I had to make a couple bits in Context pub(crate) to make this work.

I'm not sure what to call the file containing Context - opentelemetry/src/context/context_store.rs - its contents are re-exported as context::Context as previously available, but double nesting the same module name is a bad practice, and so is putting anything in mod.rs itself. Open to suggestions!

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 16.66667% with 10 lines in your changes missing coverage. Please review.

Project coverage is 79.7%. Comparing base (9d3a507) to head (f781de0).

Files with missing lines	Patch %	Lines
opentelemetry/src/context/future_ext.rs	0.0%	10 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2775   +/-   ##
=====================================
  Coverage   79.7%   79.7%           
=====================================
  Files        123     124    +1     
  Lines      23107   23107           
=====================================
  Hits       18426   18426           
  Misses      4681    4681           

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