feat: Add baggage key predicate to baggage span processor

[MikeGoldsmith]

Which problem is this PR solving?

• Closes Baggage span processor - key predicate #2166

Short description of the changes

• Add baggage key predicate function as constructor parameter to baggage span processor
• Export ALLOW_ALL_BAGGAGE_KEYS predicate
• Add unit test to verify behaviour
• Update README with examples of setting a filter

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.12%. Comparing base (dfb2dff) to head (737628b).
Report is 140 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2212      +/-   ##
==========================================
+ Coverage   90.97%   95.12%   +4.14%     
==========================================
  Files         146       10     -136     
  Lines        7492      697    -6795     
  Branches     1502      143    -1359     
==========================================
- Hits         6816      663    -6153     
+ Misses        676       34     -642     

see 137 files with indirect coverage changes

[blumamir]

I think it is very much needed. Thank you for working on this.
Added few comments

[MikeGoldsmith]

@blumamir I've updated the PR to make the predicate a required parameter, and make some of the suggested changes. Please take a look 👍🏻

[blumamir]

Thanks again for addressing my comments, this feature looks great 🥇

Added one last comment on sending a config object in the constructor

[MikeGoldsmith]

Thanks @blumamir !