Refactor PlatformState into a singleton

[imalygin]

Currently, PlatformState is an instance of MerkleLeaf and it deviates from the idea of having only three types in the Merkle tree (KV storage, queue, and singleton). It should be refactored to a singleton.

When this ticket is complete, PlatformState will be migrated to the protobuf format and accessed through the State API only.

PlatformState contains several fields to be deleted:

epochHash - we no longer support emergency reconnect
nextEpochHash - same
uptimeData - currently not utilized for anything. This is a part of a feature that was not fully implemented, and it's implementation should be reconsidered separately

The unit tests associated with this change are NOT part of this ticket. This ticket will NOT result in a mergable PR. Criteria of Done for this ticket is getting informal signoffs on the draft PR so that unit tests can be updated (another ticket) without rework.

[imalygin]

Blocked by #13084