[EFM Recovery ] Backward-compatibility for EpochCommit service event conversion logic

[jordanschalm]

Context

We are deploying EFM Recovery as a Protocol HCU, so software changes need to be backward compatible with existing state and upgrade-safe. See Upgrade Plan (Notion) for details.

This issue is for implementing backward-compatibility for the EpochCommit service event parsing logic. The new EpochCommit service event has new fields, and changes the semantics of one existing field:

• EpochCommit.dkgIdMapping field - new field added
• EpochCommit.dkgGroupKey field - new field added
• EpochCommit.dkgPubKeys field - existing field, semantics changed to omit group key

We can detect which service event type we are operating on by the existence of the new fields.

Definition of Done

• Implement conditional service event parsing logic, with one path for the old event model, and one path for the new event model
• Implement tests against both old and new service event model types

[durkmurder]

Implemented in #6795