feat: Add epoch activity REST endpoints with optional historical storage

[whankinsiv]

This PR:

• Adds current_epoch and optional epoch_history fields to the epoch_activity_counter state.
• Introduces a store-history config option in omnibus.toml to enable historical epoch activity storage.
• Implements /epoch and /epochs/{epoch_number} REST endpoints mentioned in Add REST endpoint to EpochActivityCounter #84.
• Updates openapi.yaml to document new endpoints

[sandtreader]

To avoid duplication you can reference common structures with $ref - e.g.

       content:
          application/json:
              schema:
                $ref: '#/components/schemas/EpochData'

Then later (at top level)

components:
  schemas:
    EpochData:
        type: object
        properties:
            ...
            

[whankinsiv]

I made all 200 return types into components with shared subcomponents to clean up the top level paths in the latest commit.

[sandtreader]

This could just be self.get_current_epoch() now, right? Although you'd have to clear the vrf_key_hashes separately.

[whankinsiv]

Fixed.

[sandtreader]

Feels like this could be the more natural thing to store (without the Rest suffix), and derive the EpochActivityMessage from that? Would save (duplicated) conversions below.

[sandtreader]

That was dumb - VRF keys are encoded strings in the REST version. Please ignore!