Create offer

[blabno]

Once user is able to setup payment account (#8) and obtain BTC wallet address (#9) they can create an offer.

Add an endpoint that allows creating offer. This is one of the most critical parts of the system. User input must be thoroughly validated. The validation should happen in core so it will require some core refactoring.

Initial implementation that needs to be ported is here: https://github.com/blabno/exchange/blob/api-offer/api/src/main/java/bisq/api/http/service/endpoint/OfferEndpoint.java#L109