Restrict space-only keys

[tkmsaaaam]

Resolves #2948

Someone will need to validate what the specification currently says about baggage keys and create a PR if we're not currently in compliance.

I confirmed it. (Thanks to some commenters.) Written here. This page is linked from opentelemetry-specification.

Leading and trailing whitespaces (OWS) are allowed and are not considered to be a part of the key.

So if key consists of space-only, we should treat as null.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.86%. Comparing base (2db5bb8) to head (3afc1b4).

Files	Patch %	Lines
.../api/baggage/propagation/W3CBaggagePropagator.java	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6431   +/-   ##
=========================================
  Coverage     90.85%   90.86%           
- Complexity     6030     6031    +1     
=========================================
  Files           651      651           
  Lines         17728    17728           
  Branches       1777     1777           
=========================================
+ Hits          16107    16108    +1     
  Misses         1105     1105           
+ Partials        516      515    -1     

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