feat: implement handler response on proposal execution

[nmlinaric]

Description

Implements handlerReponse in executeProposal function which is passed to ProposalExecution event.

Related Issue Or Context

Closes: #166

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 chainbridge-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

[lastperson]

1. I think we need to use abi.encode to make the data easily decodeable.
2. Wouldn't it make sense to return all the useful information from the handler if we are at it already?