cmd/k8s-operator,k8s-operator/reconciler: add PeerRelay e2e test (#20663)

This commit adds a new end-to-end test for the `PeerRelay` custom
resource.

This test is currently quite limited due to the fact that we are running
our tests within a kind cluster within github actions. This means it's
not really possible to give the peer relays a proper public IP address
via the `LoadBalancer` type services that we spin up.

That being said, we intend to expand our e2e test suite with actual
real clusters in future so this can be expanded upon at a later date.

For now, this test spins up a single and multi-replica deployment of
a peer relay and confirms that it has been registered with control
and is configured to act as a peer relay.

This test also caught a small bug where the server URL was not being
passed into the peer relay's configuration, which has been fixed here.

Closes: https://github.com/tailscale/corp/issues/45731
Closes: https://github.com/tailscale/corp/issues/45737

Signed-off-by: David Bond <davidsbond93@gmail.com>