Full asset burn

[darioAnongba]

Enable burn of full asset of anchor output (Fixes #1435, Fixes #918)

The goal of the PR is to enable "full burns" where all the asset units anchored to a UTXO are destroyed.

• Remove stop gap when the amount_to_burn equals the full UTXO amount and no other assets are present.
• The burn goes to a unique provably unspendable key.
• Ensures the burn proof is generated correctly.

This PR doesn't address issue #514, zero-value outputs accumulate and are not garbage collected.

[GeorgeTsagk]

The PR is generally heading towards the right direction

Take a look at our dev guidelines for further info around styling & git commit structure that we generally follow.

Also the linked issue seems to be referring to this as if the only we need is removing the stop gap for the full burn. I was under the impression that the stop gap was there for a protocol-level limitation related to having a simple vOutput burning all of the input amounts. cc @Roasbeef

[GeorgeTsagk]

Needs a release note that describes the updates in docs/release-notes/release-notes-0.7.0.md with a link to this PR
(this will fix the failing CI step)

Otherwise, looks good!

[jtobin]

The litd itest failures appear to be unrelated. LGTM! 👍

[GeorgeTsagk]

very close

[GeorgeTsagk]

Lgtm