evm: sourceNttManagerAddress is not validated when a partially attested message is released during a transceiver threshold decrease

[johnsaigle]

Ordinarily _verifyPeer() in the attestation flow ties the sourceNttManagerAddress to the payload indirectly. However, if:

• a message has some but not all attestations AND
• the threshold decreases during transceiver managements, AND
• someone calls executeMsg for that message with a different sourceNttManagerAddress

then the message could be processed with an incorrect manager address.

This has implications in only two places:

• The event emitted on release
• handleAdditionalPayload -- this is integrator-specific logic which the core maintainers of this project do not have control of or visibility into.

Integrators relying on sourceNttManagerAddress while handling additional payloads should be aware that this data may not be trustworthy in the unlikely edge-cases described above.