🐛 #200: Implemented a custom json per line stream reader to read Cohere chat streams correctly

[roma-glushko]

Cohere has a unique format of their streams which is not SSE but json chunk per line.

In this PR,

• I'm implementing a custom stream reader to correctly handle Cohere streams
• Start handling the stream-start event to propagate generationID to all following chunks

Testing

Changes were tested via Glide Python client:

[codecov]

Codecov Report

Attention: Patch coverage is 81.03448% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 67.34%. Comparing base (45d9aa1) to head (e2eb8ab).

Files	Patch %	Lines
pkg/providers/cohere/stream_reader.go	75.67%	8 Missing and 1 partial ⚠️
pkg/providers/cohere/chat_stream.go	90.47%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #201      +/-   ##
===========================================
+ Coverage    65.89%   67.34%   +1.44%     
===========================================
  Files           77       78       +1     
  Lines         3548     3586      +38     
===========================================
+ Hits          2338     2415      +77     
+ Misses        1085     1043      -42     
- Partials       125      128       +3     

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