[1.x] Add possibility to configure client request and response content type

[dmytro-kashcheiev]

Add possibility to configure command, query, subscription request and response content type.

This issue relates to both web and dart repositories.

In the current implementation, the Spine client sends Base64 encoded request and expects JSON response. This looks confusing during servlet implementation.

Add possibility to setup a client to work in JSON Request vs JSON response mode, or Base64 Request vs Base64 Response mode.

[armiol]

Addressed for web via #194.