FIX: Implement Msg interface for MsgInsertBTCSpvProof

[vitsalis]

ignite chain serve was failing due to the following error:

>>> ignite chain serve
panic: can't register request type *types.InsertBTCSpvProofRequest for service method /babylonchain.babylon.btccheckpoint.Msg/InsertBTCSpvProof

[stack trace]

After some investigation, this seems to be due to the InsertBTCSpvProofRequest message not implementing the Msg interface. Added that and also modified the proto files to use the package and naming convention that we use on other modules.

[KonradStaniec]

thanks for fixining this 👍 One note though, I would avoid testing stuff using ignite, as this is not something we will support, the same failure can be achieved by running local net via make localnet-start