feat: get header from sendRequestRaw

[WqyJh]

Add response header to CreateSpeech and GetFileContent.

Replace io.ReadCloser with RawResponse, which contains an io.ReadCloser and a httpHeader. The RawResponse can be treated as io.ReadCloser therefore keeping backwards compatibility.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.46%. Comparing base (0925563) to head (1b81ef8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #694      +/-   ##
==========================================
- Coverage   98.46%   98.46%   -0.01%     
==========================================
  Files          24       24              
  Lines        1366     1365       -1     
==========================================
- Hits         1345     1344       -1     
  Misses         15       15              
  Partials        6        6              

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[WqyJh]

Hi @sashabaranov, would you please checkout this PR?

[sashabaranov]

@WqyJh sorry for taking so long and thank you for the PR! Looks good to me!