Add a DNSRPS-enabled build to regular CI pipelines

DNSRPS-enabled builds have recently been silently broken a few times due to that feature not being tested in regular CI pipelines. This MR attempts to address that, but there are some pitfalls (see the rather lengthy commit messages).

The CI pipelines for this MR are expected to fail until it gets rebased on top of !7595 (merged) - showing that this MR enables DNSRPS-related breakage to be detected in advance.

Depends on !7595 (merged)

Inspired by #3827 (closed) & #3891 (closed)