[Regulator] Add Regulator Service

What does this implement/fix? Explain your changes.

[x] Add REGULATOR_PRIVATE_KEY as parameter for the container to configure a private key for the Regulator Service.
[x] Add endpoint registerPairSenderReceiver to receive client calls with { PKa, PKb, PKx, pkx PKb } parameters and result of scalar mult { pkr PKb }. This registration will store parameters in some collection transfer_secrets.
[x] Update the client service to be able to decrypt commitments for the regulator using the previous information and the commitments from transactions. Use another commitment collection commitments_regulator in the database for the regulator commitments.