Add getBlockTime RPC API

[mvines]

It's strongly desired by some users to have a network-controlled best effort timestamp associated with the production time of each block. One such example is https://en.bitcoin.it/wiki/Block_timestamp

Some important properties of this timestamp are:

1. Drift of no worse than a couple hours from the actual block production time
2. Replaying the ledger at a later time results in the same block production time
3. The block production time is not controlled by a single centralized oracle, but ideally is a function that uses inputs from all validators

A rough proposal would be to require validators to submit their current UTC time every N minutes. A median time is then constructed from the latest submission and block times are then computed as last_median_time + slot_offset * 400ms until the next median time is constructed.

This issue can be broken up into:

1. The definition/implementation of the getBlockTime RPC API
2. Initial stub implementation where last_median_time is fixed to 0.
3. Adding time submission to all validators and producing a real last_median_time

1 and 2 are important to implement quickly to unblock some users

[CriesofCarrots]

Remaining steps after #7233 :

• Calculate stake-weighted mean timestamp (https://docs.solana.com/book/v/master/proposals/validator-timestamp-oracle#calculating-stake-weighted-mean-timestamp)
• Calculate estimated block time, and return real data in getBlockTime rpc
• Incorporate timestamp expectations in validator uptime calculations

[CriesofCarrots]

Added new issue for unfinished item above: #7485