Allows to define custom headers for HTTP protocol

[Aschen]

What does this PR do ?

This PR allows to define custom HTTP headers that will be send with every request made by the SDK.

Those headers are defined when instantiating the HTTP protocol:

const kuzzle = new Kuzzle(new Http('localhost', { 
  headers: {
    'Accept-Encoding': 'gzip, deflate'
  }
}));

For example, this allows to use a different Accept-Encoding header to use gzip or deflate compression with Kuzzle

[codecov]

Codecov Report

Merging #661 (895efef) into 7-dev (1900bf0) will increase coverage by 0.08%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##            7-dev     #661      +/-   ##
==========================================
+ Coverage   85.68%   85.77%   +0.08%     
==========================================
  Files          36       36              
  Lines        1712     1715       +3     
  Branches      311      314       +3     
==========================================
+ Hits         1467     1471       +4     
+ Misses        183      182       -1     
  Partials       62       62              

Impacted Files	Coverage Δ
src/protocols/Http.ts	84.00% <91.66%> (+0.86%)	⬆️

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