Change key authorization signing to EIP-712

[legion2002]

Describe the change

Summary

-- Key authorization signatures in the protocol are currently RLP-encoded, which makes them difficult to sign with common wallets like MetaMask. We should migrate key authorization signing to the EIP-712 typed data standard so that wallets can natively sign the payload.  

Motivation

• Wallet compatibility: MetaMask and other wallets do not support signing arbitrary RLP payloads, but they already support EIP-712 typed data.
• Developer ergonomics: Using the same EIP-712 flow as other Tempo transactions simplifies integrations and reduces bespoke signing paths.
• User clarity: Typed data provides clear signing prompts, reducing risk of incorrect or opaque approvals.  

Proposal

• Define an EIP-712 domain and typed data schema for key authorizations that captures the current RLP fields (including spend limits and permissions).
• Update the keychain/key authorization flow to produce and verify EIP-712 signatures instead of RLP-encoded payloads.
• Ensure backwards compatibility plan is documented (e.g., if RLP signatures should be deprecated or temporarily supported).
• Update developer documentation and examples to reflect the new signing format.  

Acceptance Criteria

• Key authorization signatures are generated and verified using EIP-712 typed data.
• Wallets that support EIP-712 (e.g., MetaMask) can sign key authorization payloads without custom flows.
• Documentation includes the new typed data schema and guidance for integrators.
• Tests cover EIP-712 signing and verification for key authorizations.

Additional context

No response