[Proof] Relay signature & Merkle proof validation

[bryanchriswhite]

Objective

The proof module MUST validate Merkle proofs (i.e. MsgSubmitProof#closest_merkle_proof) in its msgServer#SubmitProof() method.

• May depend on [Session] Fix session hydration considering genesis has block height 1 #377.
• Integration test implementation depends on [Testing] Investigate cost/benefit of in-memory network vs. E2E tests #404.

Origin Document

Goals

• Validate that the merkle proof's root, path, value hash match on-chain expectations.
• Validate that the relay (proof value) request and response signatures match the application and supplier (respectively) for the given session.

Deliverables

• A PR which implements merkle proof validation.
• A PR which implements relay unpacking and validation.
• A PR which adds relay_min_difficulty to the proof module.
• A PR which implements relay minimum difficulty validation.

Non-goals / Non-deliverables

• Refactoring outside the scope of the goals outlined in this issue.
• Additional validation (e.g. distribution).

General deliverables

• Comments: Add/update TODOs and comments alongside the source code so it is easier to follow.
• Testing: Add new tests (unit and/or E2E) to the test suite.
• Makefile: Add new targets to the Makefile to make the new functionality easier to use.
• Documentation: Update architectural or development READMEs; use mermaid diagrams where appropriate.

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Creator: @bryanchriswhite
Co-Owners:

[red-0ne]

Closed as per #406