chore: enhance mock attestation struct with conditional validation

[DSharifi]

• closes mocked attestations should support constraints for verification #1015
• stacked on top of chore: use test_utils crate for image hashes in contract tests #1013
• Intended to merge once into main once chore: use test_utils crate for image hashes in contract tests #1013 is closed.

Changes

• renamed LocalAttestation -> MockAttestation to make it crystal clear that this is a mocked attestation.
• MockAttestation is now an enum with explanatory variants

pub enum MockAttestation {
    /// Always pass validation
    Valid,
    /// Always fails validation
    Invalid,
    /// Pass validation depending on the set constraints
    WithConstraints {
        mpc_docker_image_hash: Option<MpcDockerImageHash>,
        launcher_docker_compose_hash: Option<LauncherDockerComposeHash>,
        expiry_time_stamp_seconds: Option<u64>,
    },
}

• Added an integration test to the contract uses MockAttestation::WithConstraints to showcase how to use this enum -> new_hash_and_previous_hashes_under_grace_period_pass_attestation_verification. This test closes We should be able to re-validate a node running the old and new MPC image during the grace period, after updating the MPC docker image hash #730