[api] Context nullable annotations

[CodeBlanch]

Relates to #5801

Changes

• Adds nullable annotations for the Context API

Details

This was split off from #5801 so it could be reviewed/discussed independently.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

Attention: Patch coverage is 30.43478% with 16 lines in your changes missing coverage. Please review.

Project coverage is 86.27%. Comparing base (6250307) to head (754f100).
Report is 318 commits behind head on main.

Files with missing lines	Patch %	Lines
...elemetry.Api/Context/RemotingRuntimeContextSlot.cs	0.00%	9 Missing ⚠️
...metry.Api/Context/ThreadLocalRuntimeContextSlot.cs	0.00%	4 Missing ⚠️
src/OpenTelemetry.Api/Context/RuntimeContext.cs	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5850      +/-   ##
==========================================
+ Coverage   83.38%   86.27%   +2.88%     
==========================================
  Files         297      257      -40     
  Lines       12531    11176    -1355     
==========================================
- Hits        10449     9642     -807     
+ Misses       2082     1534     -548     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.19% <30.43%> (?)
unittests-Project-Stable	86.14% <30.43%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...emetry.Api/Context/AsyncLocalRuntimeContextSlot.cs	100.00% <100.00%> (+25.00%)	⬆️
...rc/OpenTelemetry.Api/Context/RuntimeContextSlot.cs	55.55% <100.00%> (+5.55%)	⬆️
src/OpenTelemetry.Api/Context/RuntimeContext.cs	47.72% <40.00%> (-0.10%)	⬇️
...metry.Api/Context/ThreadLocalRuntimeContextSlot.cs	0.00% <0.00%> (ø)
...elemetry.Api/Context/RemotingRuntimeContextSlot.cs	0.00% <0.00%> (ø)

... and 225 files with indirect coverage changes