[custom channels 3/3]: add FundingController to manage channel asset funding

[Roasbeef]

In this PR, we add a new sub-system, that FundingController which is tasked with managing the asset channel funding process from start to finish. The new funding sub-system will communicate directly with an lnd node (in-process or via RPC) to drive the funding process with an externally funded PSBT template. We'll interact with the local AssetWallet to funding+sign the PSBT on all levels, then ask lnd to validate what we've created, before finally broadcasting the final transaction. Before we kick off the funding process, we'll send a series of input validation proofs to the remote party. They'll validate these proofs (sent as custom messages) to ensure that we can actually sign for the assets sent.

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

TODO

• The PSBT returned from lnd will have the funding output append to it. As is, we're able to construct the funding output entirely ourselves, but the internal key isn't yet known, so the default anchoring process will arrive at the wrong output. We may need also have lnd return the musig2 internal key for this process to work properly.
• The default anchor txn logic will make a series of dummy outputs (1k sats). However, the channel may be funded with an output larger than that to be able to pay for co-op close fees. We'll need to be able to pass a custom amount. This is related to the above TODO
• Add a concrete implementation of PsbtChannelFunder

[guggero]

Awesome, looks very good! Left some suggestions and clarifications to make sure we have the same flow in mind.

[guggero]

Super exciting!

[guggero]

Ready for full review.

[ffranr]

Just one comment here to consider: #882 (comment)

[GeorgeTsagk]

✔️