[EFM] Dynamic Protocol State injects EpochExtension when entering EFM

[kc1116]

Context

The Dynamic Protocol State already detects and triggers EFM. It will also need to inject EpochExtensions when EFM is triggered, and on an ongoing basis while EFM remains triggered. When we enter EFM as a result of processing some block B, we switch to using the epochFallbackStateMachine to process subsequent blocks. Modify existing EFM triggering logic to simultaneously inject the first EpochExtension to the protocol state in block B.

This issue is for injecting the first EpochExtension (entering EFM). #5726 is for adding EpochExtensions on an ongoing basis (maintaining EFM). #5731 is for the detailed logic to avoid changing already committed leader selections.

Further Reading

Design

[durkmurder]

Implemented in #5773