record+routing/route: add AMP record

[cfromknecht]

This PR adds the currently proposed AMP record from lightning/bolts#658, and
implements the sanity checks when encoding a particular route.Hop. For now, this could will
remain dead until a number of other components are in place, but this serves as a starting point to
begin implementing the remainder of the proposal that depends on these primitive definitions
before ultimately enabling it externally.

Related to #3939.

[Roasbeef]

LGTM 🌋

No major comments, happy to see the ball start rolling once again. Even if this is just dead code atm, it'll make any subsequent changes easier as they won't need to continually rebase and manage conflicts for a series of PRs.

[cfromknecht]

fixed the linter errors, build now passes

[joostjager]

Does merging this now really make things easier? It doesn't look like this code has conflict potential. In general I think it is better to merge living code, unless we have for example two prs that build onto this. But not sure if that is already the case.