Adds Baggage.getEntry(String key)

[breedx-splk]

Resolves #6729.

This prevents users from having to go through asMap() in order to fetch the metadata for a given BaggageEntry.

[trask]

makes sense to me

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.09%. Comparing base (a6a9acb) to head (5d1fd65).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
...ain/java/io/opentelemetry/api/baggage/Baggage.java	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6765      +/-   ##
============================================
+ Coverage     90.02%   90.09%   +0.06%     
- Complexity     6488     6544      +56     
============================================
  Files           719      728       +9     
  Lines         19587    19702     +115     
  Branches       1931     1935       +4     
============================================
+ Hits          17634    17750     +116     
+ Misses         1354     1349       -5     
- Partials        599      603       +4     

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