Assure link between evm signer and signing offer

[8e8b2c]

Relates to #31

Option 1

Add a wallet_attestation_ah field to EvmSigningOffer which is validated to have the same author. Then links against an evm address can be validated against the attestation. The evm-bytes-signer micro-service would ensure the attestation exists on launch.

Pros

• Simple implementation

Cons

• (minor) Link validation requires chaining must_gets
• A compromised agent can freely reuse a wallet attestation; similarly a phished evm signature could be reused indefinitely

Option 2

Nest EvmSigningOffer inside of a SignedEvmSigningOffer. The evm-bytes-signer micro-service then signs this offer as part of creation.

Pros

• Knowledge that offers have been individually approved by the wallet owner

Cons

• Slightly more work to implement
• Confusing struct name: SignedEvmSigningOffer