Adding json_serialize and json_deserialize to requests transport

[leszekhanusz]

For the RequestsHTTPTransport:

• Allow users to overwrite the json.dumps method used to serialize the json sent to the backend.
• Allow users to overwrite the json.loads method used to deserialize the json received from the backend.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (a3f0bd9) 100.00% compared to head (1ae00ab) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #466 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 29 29 Lines 2625 2629 +4 ========================================= + Hits 2625 2629 +4 ```

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