In this commit, we convert the `JusticeKit` struct to an interface.
Then, we add two implementations of that interface:
1) The `legacyJusticeKit` which implements all the methods of
`JusticeKit`
2) The `anchorJusticKit` which wraps the `legacyJusticeKit` and just
re-implements the `ToRemoteOutputSpendInfo` method since.
In this commit a new enum, CommitmentType, is introduced and initially
there are 3 CommitmentTypes: Legacy, LegacyTweakless and Anchor.
Then, various methods are added to `CommitmentType`. This allows us to
remove a bunch of "if-else" chains from the `wtclient` and `lookout`
code. This will also make things easier to extend when a new commitment
type (like Taproot) is added.
