Add Support for Multisig Transactions

[GAtom22]

Proposal: Support for multi-signature transactions is germane in the blockchain ecosystem. To provide Ethermint users with this key feature, this issue proposes to add the required functionality.

Current behavior: When trying to perform a multi-sig transaction, the application returns an invalid pubkey error (unrecognized key type: *ethsecp256k1.PubKey: invalid pubkey)

Desired behavior: Multi-sig transactions should be processed as expected, applying the corresponding change in the application state.

Use case: Multi-sig transactions has a number of applications:

• Dividing up responsibility for possession of bitcoins among multiple people.
• Avoiding a single-point of failure, making it substantially more difficult for the wallet to be compromised.
• m-of-n backup where loss of a single seed doesn't lead to loss of the wallet.