feat(context): add w3c() helper for W3C trace fields

[maxday]

Adds Context.w3c() which exposes allowlisted W3C trace-context fields (traceparent, tracestate, baggage) carried on clientContext.w3c, and strips w3c from clientContext so callers can only read trace fields through the helper.

• aws-lambda-java-core: new default Context.w3c() returning an empty map (backward compatible for existing Context implementations).
• RIC: LambdaContext reads/allowlists/strips w3c off the client context at construction and returns an unmodifiable map; non-object payloads and non-string values are dropped.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.79%. Comparing base (e38423d) to head (c6aafd0).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #657      +/-   ##
============================================
+ Coverage     66.03%   66.79%   +0.76%     
- Complexity      214      223       +9     
============================================
  Files            35       35              
  Lines           998     1018      +20     
  Branches        143      146       +3     
============================================
+ Hits            659      680      +21     
+ Misses          287      286       -1     
  Partials         52       52              

Flag	Coverage Δ
aarch64	66.79% <100.00%> (+0.76%)	⬆️
x86_64	66.40% <100.00%> (+0.77%)	⬆️

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