Define the configuraiton options for the DID module

[noandrea]

To make the module generic so it can be included in the cosmos-sdk it is important that some key aspects are configuratble:

• did method name (currently cash)
• supported verification relationships
• enable/disable ephemeral DIDs

[noandrea]

did method name configurable in genesis. In an IBC word the DID method should be unique (not sure)

relationships maybe only in node configuration? it has the risks that the did cannot be transfer through nodes / chains

[noandrea]

it has been decided not to implement this feature for the time being, it may be implemented in a future release