Both the trader offering to buy a sidecar channel for another node as
well as the recipient node will need to store the involved sidecar
tickets in their local databases. We add a simple store interface and
its implementation that will be used in future commits.
For proper and safe channel acceptance the recipient node also needs to
know which key index it used for a ticket's multisig key so it can sign
the authentication flow with the auctioneer correctly (which takes a key
locator and not key descriptor).
To make the offer and sidecar contract more explicit, we also include
the lease duration in blocks in the offer part. This field will
overwrite any value set when using the CLI to submit the bid order.
With this commit we add the main sidecar Ticket type and the partial
information types that are bundled within.
The ticket itself is meant to be used in a PSBT like manner where each
participant incrementally adds their information according to their role
and the current step.
