chore: Finality Provider registration with signer as fp address

[RafilxTenfen]

Remove signer and babylo_pk from MsgCreateFinalityProvider to use the signer of the msg as the finality provider address directly:

 message MsgCreateFinalityProvider {
-  option (cosmos.msg.v1.signer) = "signer";
- -  string signer = 1;
+  option (cosmos.msg.v1.signer) = "fp_addr";
+  string fp_addr = 1 [(cosmos_proto.scalar) = "cosmos.AddressString"];

-  bytes btc_pk = 5;

-  ProofOfPossession pop = 6;
+  ProofOfPossessionBTC pop = 5;
 }

Since the structure ProofOfPossesion will not be used anymore to check the Babylon public key it can also be removed from the code

Refs:

• https://github.com/babylonchain/babylon/pull/657
• https://github.com/babylonchain/pm/pull/38

[RafilxTenfen]

Done at #678