Implement `relayMessage` and `waitForMessage`

[Dhaiwat10]

Also, make improvements to the withdraw to base layer tests.

Refer to the bridge repo:

• https://github.com/FuelLabs/fuel-bridge/

Important for mainnet.

Note

• Incoming from Feedback - Frontend #1537

[arboleya]

@Dhaiwat10 Do you remember what this is about, and is it something we should still look into?

[Dhaiwat10]

@arboleya I faintly remember these two being some helper/utility methods in the bridge codebase. Not sure if it is something that we still need to work on. Might be worth asking the frontend/bridge team

[arboleya]

It looks like a lot changed in the bridge, let's close for now and reopen if necessary.