feat: implement client get_metadata for requirement 1.2.2

[federicobond]

This PR

Implements requirement 1.2.2:

The client interface MUST define a metadata member or accessor, containing an immutable name field or accessor of type string, which corresponds to the name value supplied during client creation.

[codecov]

Codecov Report

Merging #141 (1394102) into main (292a0df) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #141      +/-   ##
==========================================
+ Coverage   92.91%   93.02%   +0.11%     
==========================================
  Files          20       20              
  Lines         367      373       +6     
==========================================
+ Hits          341      347       +6     
  Misses         26       26              

Flag	Coverage Δ
unittests	93.02% <100.00%> (+0.11%)	⬆️

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Impacted Files	Coverage Δ
open_feature/open_feature_client.py	95.87% <100.00%> (+0.27%)	⬆️

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