Support protocol discover-features

[FabioPinheiro]

Add support for the protocol discover-features 2.0

https://didcomm.org/discover-features/2.0/

The specification can be found at here

Value brief

• Show users what services/protocols are supported.
• Applications can automatically adapt the intention to the protocols that are supported.

Implementation notes

The Mediator should replay to messages of the type https://didcomm.org/discover-features/2.0/queries with messages of the type https://didcomm.org/discover-features/2.0/disclose.

Since there is no reason for the Mediator to hide what protocols are supported.
The disclose MUST include all the support protocols (except itself?).

We should use it encrypted message since this is what is expected by most applications at the moment.
But would be nice to have a signed message that can be exposed on the web page.

[FabioPinheiro]

The work here is done.
Would also be nice to add to the mediator test suite.

[FabioPinheiro]

fyi I created a ticket on the Mediator test suite for that.
input-output-hk/didcomm-v2-mediator-test-suite#9