[custom channels 2/3]: add aux leaf signer

[guggero]

Depends on #878.

Implements a first version of the aux leaf signer.

Link to all PRs in the saga:

• 1/3: [custom channels 1/3]: create aux leaf store and tlv.Blob decoders #878
• 2/3: [custom channels 2/3]: add aux leaf signer #885
• 3/3: [custom channels 3/3]: add FundingController to manage channel asset funding  #882
• 4/3: [custom channels 4/3]: add traffic shaper and invoice manager implementations #912

[ffranr]

Some simple CI lint errors, otherwise 👍

[ffranr]

Use txscript.SigHashType here instead of uint32?

[guggero]

This would lead to panic: unknown primitive type: *txscript.SigHashType [recovered]. We either need to have types that have a Record() method or are from a list of defined primitive types (no type aliases allowed).

[GeorgeTsagk]

Lgtm 🥬

[GeorgeTsagk]

nit: why not follow lnd codebase and use a "maybe" fn.Option[AuxLeafSigner] ?

[guggero]

We're always creating and starting these components. Since they're pretty light-weight, it IMO doesn't make sense to only conditionally create and start them for the custom channel specific use case, as that would mean we'd need to deal with more .WhenSome() and .IsSome() constructs.

But maybe something to consider in this regard is to only start the HTLC and invoice acceptors if they're really needed, since both of them can only be registered once in lnd. So if we always occupy that single "slot" with tapd even if you don't use custom channels, that could create issues for some users. I'll look into that in the invoice PR.