[Relayer] Implement dedicated relay store to optimize memory usage

[red-0ne]

Summary

Adds persisted relay store to decouple relay data from SMST storage

Primary Changes:

• Introduces RelayStore interface for persisting relay data separately from SMST
• Implements file-based relay storage with buffered I/O optimizations
• Modifies proof submission to include full relay data for signature verification

Secondary Changes:

• Updates proto definitions to include proof_relay field
• Adds new error types for relay storage operations
• Modifies test utilities to handle relay proof verification

Issue

• Description: Currently, the relayer's SMST stores the complete relay data within its nodes in memory alongside the SMST leads to high memory usage. Considering that relay can be several kilobytes in size.
• Issue: [Load Testing] 10B Relay Load Test #1106

Type of change

Select one or more from the following:

• New feature, functionality or library
• Consensus breaking; add the consensus-breaking label if so. See [Infra] Automatically add the consensus-breaking label #791 for details
• Bug fix
• Code health or cleanup
• Documentation
• Other (specify)

Sanity Checklist

• I have updated the GitHub Issue assignees, reviewers, labels, project, iteration and milestone
• For docs, I have run make docusaurus_start
• For code, I have run make go_develop_and_test and make test_e2e
• For code, I have added the devnet-test-e2e label to run E2E tests in CI
• For configurations, I have update the documentation
• I added TODOs where applicable