Add encoding of APK and Signature for contract readability

[waymobetta]

Description

In order to further validate proofs on the contract side, some additional encoding of SignatureVerification struct fields is required.

This PR specifically targets the following fields:

• APK: Aggregated Public Keys of the validators
• Signature: The signature of the block being proved (extracted from the Istanbul Extra data)

Note: In order for this to be tested either Celo PR #23 needs to be merged or the go.mod needs to be modified to replace the celo-bls-go package with the cloned, local version of the referenced celo-bls-go package with the branch kobigurk/arkworks checked out.

Related Issue Or Context

Closes: #120

How Has This Been Tested? Testing details.

Types of changes

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

Checklist:

• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have ensured that all acceptance criteria (or expected behavior) from issue are met
• [ ] I have updated the documentation locally and in chainbridge-docs.
• [x] 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

[lgtm-com[bot]]

This pull request introduces 2 alerts when merging 43c28ea528f605d604ae21a6eb44c556b70d80e0 into cd8bf832b9b64380cb298ef78af1fd2aa92f3041 - view on LGTM.com

new alerts:

• 2 for Useless assignment to local variable