A sidecar ticket will always use the minimum version needed to encode
all the relavant information for that offer. In that way the tickets
from a provider with a newer version can be redeemed by recipeients
using older versions as long as the ticket does not encode new
functionalities.
In this commit, we change the encoding of the sidecar ticket from base68
to base58 as it doesn't contain any special characters which makes copy
pasting a string easier.
An example encoding of the new string looks something like:
```
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
```
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.
We'll need to check the self chan balance (a.k.a. push amount) values in
several places, including the upcoming sidecar ticket functionality
itself. That's why we add these check functions to the sidecar package.
To allow a ticket to be transported easily over any communication
channel we also provide an easy ascii compatible exchange format which
includes a prefix and a checksum in addition to the tlv stream data.
The string encoding chosen is URL safe base64.
To allow the ticket to become a versatile and extendable data exchange
format, we encode its main parts using different TLV streams. That way
more parts can always be added or extended individually.
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.
