Include oracle announcement ID in maker's offer

[thomaseizinger]

The oracle announcement that is "to be used" needs to be included in the offer. It is part of the contract terms the taker has to agree to.

When a taker's order is accepted by the maker, the taker needs to obtain the nonces of the particular event ID from the oracle.

[thomaseizinger]

Implementation notes:

Previously, we used to not do any sort of request-response messages between actors. For this particular integration though, I think it is more useful if we have a message that can be sent to the oracle::Actor for obtaining the nonces via the xtra::Message::Result. The taker_cfd::Actor would send this message before the contract-setup is started (that is when the nonces are needed).

Currently, the scaffholding that has been done regularly updates the taker_cfd::Actor with nonces for particular event IDs. That creates an unnecessary indirection IMO.

Note that for the maker, I think it still makes sense for the oracle::Actor to update the maker_cfd::Actor with the next or upcoming attestation events that are available. If we ever expose this choice on the UI, it might make sense to also switch the maker_cfd::Actor to a request-response model here.