Mediator : Expose AddKeyToRouter function through Client

[rolsonquadras]

Currently, the mediator client provides functions to register/unregister router and to get the router config.

There can be cases where clients can create the DIDDocument and would need to add the newly created keys to the router. The function is already available in mediator service. This needs to be exposed as part of client/command layer.

https://github.com/hyperledger/aries-framework-go/blob/b3f5204cb72a7a94b157e9d815244d34b3aa1d5d/pkg/didcomm/protocol/mediator/service.go#L634