EP: Post-CD audit feedback, etc

[dorothy-zbornak]

Description

DO NOT MERGE Keeping this in a separate PR for now to not trip up consensys while they do their thing in the other one (#2657). Once they're through, this PR will get closed and #2657 will point to this commit.

• Address feedback from consensys audit.
• Add meta-transactions + signed calldata integration tests.
• Misc singing tooling bug-fixes.
• Rebase with development (TODO)

Testing instructions

Types of changes

Checklist:

• [ ] Prefix PR title with [WIP] if necessary.
• [ ] Add tests to cover changes as needed.
• [ ] Update documentation as needed.
• [ ] Add new entries to the relevant CHANGELOG.jsons.

[coveralls]

Coverage decreased (-0.08%) to 79.548% when pulling 9b6703398ea99c3700d877bc5d30c39a82d47675 on feat/exchange-proxy/post-cd-audit into 10aceb164d2cb9f6f75a845e0013e4b262c6a0f7 on fix/ep/meta-transactions.