Add proven_on_l2 finality status

[ArielElp]

A few months ago we had a mainnet reorg that was caused by a sequencer/prover mismatch. This new status (pending decision to add it to the SN) will reflect the successful execution of the Starknet OS. This will give a much higher degree of confidence to parties that are now waiting for accepted_on_l1 (e.g. some exchanges). While not really reflecting proof being created, the new status captures all the known issues we had so far (os/blockifier mismatch).

The new status is added to:

• TXN_STATUS (returned on the getTransactionStatus endpoint)
• TXN_FINALITY_STATUS (included in transaction receipts)

[ArielElp]

Closing for the time being as adding such a status involves a deeper change in the flow of full nodes, forcing them to periodically query the centralized sequencer for old blocks to see whether or not their status has changed.