Implement BOLT2: `opt_shutdown_anysegwit`

[ariard]

See lightning/bolts#672.

[TheBlueMatt]

We should probably do a few things related to this - we should (a) implement the protocol itself, (b) we should require the feature for all peers, (c) we should change KeysInterface.get_shutdown_pubkey to allow it to return any segwit script, probably via a segwit type enum or so.

[ariard]

Closed by #794