Oracle selection protocol

While we are still using message passing we could do programmable oracle selection by putting oracle address in the extra data of a deal. The default alternative is that there is one prover contract for each onramp contract so SPs could make multiple deals with the same data to satisfy multiple different cross chain oracles.

We could also notify multiple oracles for a single deal i.e. half the data paid for by polygon half by matic.

This might not be that important since it probably goes away once we move from message passing to state oracle.

No files, tests, or entry points are named. Start by reviewing the current message-passing flow and how deal extra data, prover contracts, and oracle notifications are represented. Define the desired oracle-selection behavior and confirm whether it remains relevant before implementing or testing it.

Written by the indexing model from the issue text.