Master Secret / Link Secret structure

[genaris]

Based on our initial experience working with anoncreds-rs in AFJ, I noticed that, when converted from/to JSON, the MasterSecret object has an structure that does not seem related to anything expressed in AnonCreds spec.

It currently follows the form:

 { value: { ms: string } }

I guess this comes from some format used in former Indy SDK, but IMO it should be a plain string, so there would not be need of JSON conversion methods and would be clearer from API perspective.

[genaris]

This has been solved by #153