Add CommitedSeal data to SignatureVerification

[waymobetta]

WIP

Description

CommitedSeal data is needed for additional BLS signature verification of the block.

• Added CommitedSeal struct
• Stored this struct as pointer within SignatureVerification struct
• Added utility functions to construct the CommitedSeal data:
• [x] CommitedSealSuffix
• [x] CommitedSealPrefix
• [x] CommitedSealHints

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.

Additional Note: This also contains code from the current Add encoding of APK and Signature for contract readability PR currently in progress. It is assumed that that PR will be merged prior to this one.

TODO:

• The listener block is getting quite large due to all of this data encoding that needs to be done. Consider refactoring and moving the bulk of this into a helper function.
• Much of this functionality is currently stored within the utils package; consider breaking out into its own package.

Related Issue Or Context

Closes: #119

How Has This Been Tested? Testing details.

Added tests for:

• [x] TestCommitedSealSuffix
• [x] TestCommitedSealPrefix
• [x] TestCommitedSealHints

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