orders: add a new MustMatch order matching predicate that allows directed orders

It would be useful to add a _new_ order matching perdicate, called `MustMatch` (as an examlpe), that takes a _list_ of public keys and only allows an order to be matched if the other side of the match is amongst that set of public keys. This allows users to attempt to purchase/sell channels to/from an _specific_ node. Such a feature can be used as an example to create a _market driven autopilot_ agent that attempts to greedily maximize a metric such as betweenness centrality by purchasing inbound channels from the set of N nodes that will serve to maximize its personal bt centrality gain.

No files, tests, or entry points are named. Start by locating the existing order-matching predicates and their tests, then define the MustMatch behavior around a public-key allowlist and verify that matches occur only with listed peers.

Written by the indexing model from the issue text.