New intention precedence for external intentions.

Support external intentions by making source type part of the precedence
decision. The goal is that an external-uri intention, e.g.
spiffe://trust.domain/path, is ranked higher than an
external-trust-domain intention, e.g. spiffe://trust.domain, because it
is a more exact match.

The maximum precedence value is increased, however the relative ordering
of Consul intentions has not changed.