Chore Add stream headers and header specific methods

[henomis]

Describe the change
This branch:

• adds header specific methods
• adds headers for streams
• adds headers documentation in README

Describe your solution
httpHeader struct implements specific methods to easily get information. The same type has been embedded into streams.

Tests
Tests have been integrated testing new methods

issues:

• Return x-ratelimit-* header #495
• Return response headers too #452

Related PR:

• Feat Add headers to openai responses #506

[codecov]

Codecov Report

Merging #510 (72ee180) into master (8e165dc) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #510      +/-   ##
==========================================
+ Coverage   97.52%   97.56%   +0.04%     
==========================================
  Files          18       18              
  Lines         850      864      +14     
==========================================
+ Hits          829      843      +14     
  Misses         15       15              
  Partials        6        6              

Files	Coverage Δ
chat_stream.go	100.00% <100.00%> (ø)
client.go	100.00% <100.00%> (ø)
stream.go	100.00% <100.00%> (ø)

[henomis]

Closed beacusa it's a duplicated. Implemented here: #507