adding baggage setValue and removeValue, plus example usage

[brettmc]

setValue and removeValue were added to spec after our initial baggage implementation, so implement the missing features.
ref: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/baggage/api.md#set-value
ref: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/baggage/api.md#remove-value

[codecov]

Codecov Report

Merging #888 (74ef563) into main (587bc5a) will not change coverage.
The diff coverage is n/a.

❗ Current head 74ef563 differs from pull request most recent head 4ba14b0. Consider uploading reports for the commit 4ba14b0 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #888   +/-   ##
=========================================
  Coverage     81.56%   81.56%           
  Complexity     2008     2008           
=========================================
  Files           264      264           
  Lines          5219     5219           
=========================================
  Hits           4257     4257           
  Misses          962      962           

Flag	Coverage Δ
7.4	81.05% <ø> (ø)
8.0	81.55% <ø> (ø)
8.1	81.69% <ø> (ø)

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[Nevay]

I don't think we need this - functionality is already available via ::toBuilder().

[pdelewski]

I don't think we need this - functionality is already available via ::toBuilder().

Seems you are right as specification gives some freedom how and where this functionality has to be provided.

Depending on language idioms, a language API MAY implement these functions by using a Builder pattern and exposing a way to construct a Builder from a Baggage