Add required security configuration

[gjermundgaraba]

For each chain/client that is going to be attested to, we need to have a configuration setup where we define the required security.

Some discussion is needed here on the parameters:

• % of total validator set's voting power?
• number of tokens behind voting power?

This needs to be implemented in:

• the client registration and update message (implemented in Register attestation client and limit updates to registered targets #54).
• The AttestatorsHandler/Controller's SufficientAttestations method should use the configuration to check if we have enough attestations for the client.