Define update interface to ledger

Consensus relies on parts of the ledger code in order to determine the time of forks. However, this is at present done by digging into parts of the ledger state that are not guaranteed to remain invariant across eras. We should instead define a suitable API to allow consnesus to query the ledger concerning upcoming updates, and implement this for the various ledger eras.

Note that PR input-output-hk/ouroboros-network#2888 includes a partial response to this, but is unsuitable for long-term use - it just makes the current digging into the ledger state explicit.