Export more interfaces for Typescript

[Aschen]

What does this PR do?

Export more interfaces to offer more flexibility to developers when they are using the SDK.

Also the Document class has been modifier to be able to extend it:

interface AlertContent extends DocumentContent {
  name: string;

  description: string;

  // _kuzzle_info is included from DocumentContent
}

class Alert extends Document {
  public _source: AlertContent;

  constructor () {
    super();
  }
}

[codecov]

Codecov Report

Merging #556 (dddd238) into 7-dev (7d41dc3) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            7-dev     #556      +/-   ##
==========================================
- Coverage   88.39%   88.38%   -0.01%     
==========================================
  Files          32       32              
  Lines        1448     1447       -1     
  Branches      252      252              
==========================================
- Hits         1280     1279       -1     
  Misses        119      119              
  Partials       49       49              

Impacted Files	Coverage Δ
src/controllers/Realtime.ts	90.47% <ø> (-0.15%)	⬇️

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