Register attestation client and limit updates to registered targets

[gjermundgaraba]

Currently, the vote extensions doesn't really care if the client updates are registered/configured.

To limit potential for abuse and make it easier to configure each attestation target separately, they should be registered on-chain in the configmodule. This is also where configuration can be set up on things like required voting power (but that will be implemented in #55).

Implementation notes:

• We also need a way to update a target/registration (naming here might need some thought)
• We will need GPRC queries (get one, get all with pagination)
• Check in vote extensions if an incoming client update is registered
• The AttestorHandler/Controller will need a new method for checking if a client is registered
  • The SufficientAttestations method should also take in the client id for the attestations it is being asked.