[3/?]: multi: add new AuxFundingController for custom external funding flows

[Roasbeef]

Depends on https://github.com/lightningnetwork/lnd/pull/8684.

In this commit, we add the AuxFundingController interface that can be used to implement a class of custom funding channels that only requires some additional data to be inserted into the tapscript leaves for the funding/HTLC/commitment outputs. This controller also implements the MsgEndpoint interface (see https://github.com/lightningnetwork/lnd/pull/8520) as well. This allows the controller to intercept certain custom messages related to the funding process (think extra sigs for a channel factory), and even the existing funding messages.

We then create a new top level config entry for this interface. This allows an external program to instantiate a AuxFundingController, make a MsgRouter, register that, then pass both into the lnd top level config.

We also pick up where the prior PR in this series left of by using the new abstraction to map a pending chan ID (kinda like the FundingStateStep RPC) to the aux funding data. This data is then used to populate all the new custom blobs that we'll store for these new channel types, which impacts the way we construct the funding output, and also the commitment/HTLC outputs+scripts.

Link to all PRs in the saga:

• 1/?: https://github.com/lightningnetwork/lnd/pull/8683
• 2/?: https://github.com/lightningnetwork/lnd/pull/8684
• 3/?: https://github.com/lightningnetwork/lnd/pull/8622
• 4/?: https://github.com/lightningnetwork/lnd/pull/8632
• 5/?: https://github.com/lightningnetwork/lnd/pull/8641

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[github-actions[bot]]

Pull reviewers stats

Stats of the last 30 days for lnd:

	User	Total reviews	Time to review	Total comments
	guggero 🥇	25 ▀▀▀	2d 21h 27m	57 ▀▀▀
	ellemouton 🥈	17 ▀▀	4d 14h 51m ▀	34 ▀▀
	bhandras 🥉	10 ▀	30m	0
	Roasbeef	6 ▀	7h 40m	12 ▀
	yyforyongyu	6 ▀	3h 18m	0
	ProofOfKeags	4	7d 8h 54m ▀	42 ▀▀
	GeorgeTsagk	4	2d 20h 31m	10 ▀
	ziggie1984	4	3d 11h 1m ▀	9
	saubyk	3	3h 43m	0
	Chinwendu20	3	18d 5h 35m ▀▀▀	10 ▀
	hieblmi	2	21m	1
	bitromortac	2	9h 44m	0
	positiveblue	1	4d 21h 57m ▀	0
	Crypt-iQ	1	7d 14h 36m ▀	4
	dstadulis	1	1d 22h 10m	4
	morehouse	1	2d 2h 39m	1
	carlaKC	1	12h 2m	0
	mohamedawnallah	1	1d 4h 44m	0

[Roasbeef]

Pushed a few new commits: we'll now include the taproot internal key in the PBST template we return for the PSBT funding flow.

[lightninglabs-deploy]

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

[guggero]

Updated this PR to depend on #8660 so we have everything we need in one continuous branch. @ffranr and @GeorgeTsagk please skip the first 6 commits when reviewing this PR!