Add first POC for starknet msg_verifier

[eloylp]

Work from https://github.com/eigerco/giza-axelar-starknet/issues/15

Description

It includes:

• Event handler for verifying messages.
• Unit tests for the handler with mocks.
• Registers the starknet handler in handlers architecture.

Todos

• Ensure we are interacting with starknet in the correct way while validating messages.
• Ensure we are using the correct types like Message (integration tests should shout about this)
• Integration tests to ensure everything works.

[eloylp]

I guess we can close this. re-open if necessary.