[IOpenCDM] Update with JSON-RPC interface

[VeithMetro]

After merging this PR, we have to also merge this PR with changes to the plugin

[VeithMetro]

@pwielders what do you think about making keySystem an enum instead of a string? We talked about that with Sebastian, and have arguments for both.

On one hand, the CDMs list is finite and sending an enum would be more efficient, but on the other, OpenCDM don't need to know what DRM types exist

[pwielders]

@VeithMetro on you question. Agree and do have a preference for enums, however the interface specification was initially once drafted by the Frauenhofer Institute and a s long that there was no severe technical reason to deviate from what they specified, we did not modify it. So for me, as it is is fine but do agree that for a communication overhead the enum would be much better. So for sake of backwards compatibility with what Frauenhofer once specified, I vote for keeping it a string..