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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
```
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.
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.
