Make prev_tx optional during contract negotiation

[nkohen]

Full nodes and other clients with access to a full utxo set do not need to receive prev_tx when they are learning their counter-party's funding inputs because a txid and vout will suffice for them to query for this prev_tx (and txid + vout totals 36 bytes whereas a full transaction can be much much larger). Light clients which do not have access to the utxo set should still receive prev_tx.

We need to add an option during contract negotiation for parties to specify which kind of client they are so that clients with access to the full utxo set can save significant space when receiving an offer/accept message. Additionally a new funding input TLV type will be needed for non-prev_tx communication.

[NicolasDorier]

So basically, to explain my goal with this:

1. When taproot comes, the prev_tx is useless in any case,
2. A client with full utxo set access don't need it,
3. In BTCPay Server, while we will try to include it if we can, our wallet don't have the previous tx if it has been restored (as it relies on pruned node and scantxoutset). So our goal is to make the best that we can: If we have, include previous tx, if not, then not including it. If we don't have the tx, this still might be fine for 1 or 2 clients.