Add public key type

[rocco-piovani]

Prerequisites

• I checked the documentation and made sure this feature does not already exist.
• I checked the existing issues to make sure this feature has not already been requested.
• I have read the code of conduct before creating this issue.

Problem

I want to use this library to build transactions for Islamic Coin (ISLM) but the key type (/ethermint.crypto.v1.ethsecp256k1.PubKey) doesn't seem to be supported.

Feature / Solution

Implement the ethsecp256k1 public key type.

Alternatives

No response

Additional Context

No response