feature: add outbound-liquidity auction market, bidder pays for channel acceptance

[positiveblue]

Allow the submission of orders for different auction markets. Expand the current auctions, bidder pays for inbound liquidity, with another market where the bidder pays for channel acceptor, outbound liquidity.

To be able to participate in the new auctions:
The asker needs to:

• Set auction type to BTCOutboundLiquidity
• Set order amt to 100K (BaseSupply)
• Set min_chan_size to 100k or more. This will take into account the bidders funding supply.

The bidder needs to:

• Set auction type to BTCOutboundLiquidity
• Set order amt to 100K (BaseSupply)
• Set self_chan_balance to 100k or more.

Sidecar tickets are not supported yet for this new market.

NOTE: Only the last 5 commits need to be reviewed in this PR

[guggero]

Did a quick pass, looks almost ready. Nice go get this with such a small diff.

[dstadulis]

after we get ZC and unannounced channels merged, we'll need 2x review on this with Roas to cap it out

[guggero]

Should be rebased on the compressed/combined ZC PR.

[lightninglabs-deploy]

@positiveblue, remember to re-request review from reviewers when ready

[guggero]

Got a few more comments, but we're very close!

[guggero]

LGTM, great work 🎉

[Roasbeef]

Did an initial pass, but tbh I'm still working to get an accurate mental model in my head of: the way the new feee accounting works, and scenarios in which a buyer/seller would want to enter into this new engagement.

Would be it be accurate to say that (assuming a fixed number of connection slots), this lets a party sell/auction off an inbound (from their PoV) connection/channel slot?

[Roasbeef]

In most cases though, we expect the channel amount to be nothing, and the actual value being the self channel balance right?

[positiveblue]

Exactly, for the BTCOutboundLiquiditythe chan amount needs to be always 100k (1 unit). This way we can roll it out and check the demand for this feature without having to make big refactor

[ffranr]

This error message talks about amount whereas the if conditional examines units. I expected something like:

if o.Details().AuctionType == order.BTCOutboundLiquidity &&
	o.Details().Amt != order.BaseSupplyUnit {
        return fmt.Errorf("to participate in the outbound liquidity " +
			"market the order amt should be exactly the base supply amount (%v sats)", order.BaseSupplyUnit)
}

Does that seem reasonable?

EDIT: FYI, I've just noticed that testing on amt would mirror this change:
https://github.com/lightninglabs/pool/pull/390/files#diff-60f9a53c1dcbdd7baee29e9e4ae17725d00da191f07abcc50f9e3c035c87c247R509

[ffranr]

I think these lines are correct. I suppose that if the bid is for oubound liquidity only a single unit will be filled. I wound consider adding a comment.

Further, I wander whether a third case is necessary:

		case ourOrder.Details().AuctionType == BTCOutboundLiquidity &&
			unitsFilled == 1

[ffranr]

I've left a few comments around the parts which I'm uncertain about.

[Roasbeef]

LGTM 🐞

[Roasbeef]

great description 👍