Allow imposing constraints on RPKI trust anchors

Allow imposing constraints on RPKI trust anchors

The ability to constrain a RPKI Trust Anchor's effective signing
authority to a limited set of Internet Number Resources allows
Relying Parties to enjoy the potential benefits of assuming trust,
within a bounded scope.

Some examples: ARIN does not support inter-RIR IPv6 transfers, so
it wouldn't make any sense to see a ROA subordinate to ARIN's trust
anchor covering RIPE-managed IPv6 space. Conversely, it wouldn't
make sense to observe a ROA covering ARIN-managed IPv6 space under
APNIC's, LACNIC's, or RIPE's trust anchor - even if a derived trust
arc (a cryptographically valid certificate path) existed. Along these
same lines, AFRINIC doesn't support inter-RIR transfers of any kind,
and none of the RIRs have authority over private resources like
10.0.0.0/8 and 2001:db8::/32.

For more background see:
https://datatracker.ietf.org/doc/draft-snijders-constraining-rpki-trust-anchors/
https://mailman.nanog.org/pipermail/nanog/2023-September/223354.html

With and OK tb@, OK claudio@