Support multiple beacon types in signer/aggregator

[jpraynaud]

Why

The Cardano transactions should be signed given a beacon based on Block range, and it should cohabit with the existing system of beacons based on Epoch/Immutable file number. The current design is based on a beacon specific to immutable files and is not versatile enough to support all the upcoming types.

Note: The chain observer will probably need to retrieve the current block number as a new function in the trait

What

We need to adapt the structure of the beacon to support the addition of one or multiple types that will define points on the chain with different semantics. This will have a high impact on the code base and we want to avoid breaking changes.

How

• Make a PoC or exploratory version where we can assess the amount of work, difficulties, blockers:
  • Use an enum for Beacon and one of its variants is ImmutableFile?
  • Use a trait to represent a Beaconable?
  • Use a generic Beacon?
  • Create a beacon specific to the state machine?
  • Reuse existing SignedEntityType enum ?
• Use updated version for the new deprecated tags in rust
• Plan deployments:
  • Breaking changes
  • Manual updates