Support for DID PRISM

### Background
* Existing DID Peer is static; identities cannot be modified post-creation, as you may have to do eg when changing home address
* DID Prism is modifiable (it lives on the Cardano Blockchain), so needs to be supported
* We will continue to support DID Peer for compatibility (ie this is not a breaking change)

### DoD
* All interactions of the Mediator work with a DID Prism as they have previously worked with a DID Peer

#### How to Test
- Set up Mediator to use PRISM DID only
- E2E Test Suite stays green (tested against short URIs only)
- Integration Test that "returns correct DID document for the both short & long form URIs" is green

### Implementation Details/Hints
1. Add support for did:prism by adding a resolver for DID PRISM.
2. Use a did:prism DID as the agent/identity in the mediator.
3. Integration Test in DoD needs to be created