Add custom properties to a KuzzleRequest

[jenow]

What does this PR do ?

Override the operators [] and []= to KuzzleRequest object to be able to add custom properties to the request outside of the body.

How should this be manually tested?

final request = KuzzleRequest(controller: 'test', action: 'test', body: {});
request['custom'] = 'foo';
final res = await kuzzle.query(request);

The custom field should be sent to kuzzle in the request.

Other changes

Add the documentation for KuzzleRequest