Assure link between evm signer and signing offer

Relates to https://github.com/holochain-open-dev/holoom/issues/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_get`s
- 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`