feat: across message handler

[mpetrun5]

Add across message handler that verifies the across message and starts the signing process for it. The final signature will be changed after the liquidity pool solidity contract is merged.

Description

Related Issue Or Context

Closes: #10

How Has This Been Tested? Testing details.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Checklist:

• I have commented my code, particularly in hard-to-understand areas.
• I have ensured that all acceptance criteria (or expected behavior) from issue are met
• I have updated the documentation locally and in docs.
• I have added tests to cover my changes.
• I have ensured that all the checks are passing and green, I've signed the CLA bot

[github-actions]

Go Test coverage is 57.5 %\ ✨ ✨ ✨

[github-actions]

Go Test coverage is 57.5 %\ ✨ ✨ ✨