Implement Slot <-> Time conversion

[golddydev]

Description

In order to to Phase 2 validation, we need to convert Slot (validity_start, ttl) to PosixTime.

We need to have EraHistory which stores each era's summary with SystemStartTime.

Acceptance Criteria

• Make EraHistory struct to manage Slot <-> Time conversion.
• Make era_state module to store EraHistory and publish it to downstream module.