* ipn/ipnlocal,envknob: add some primitives for HA replica cert share.
Add an envknob for configuring
an instance's cert store as read-only, so that it
does not attempt to issue or renew TLS credentials,
only reads them from its cert store.
This will be used by the Kubernetes Operator's HA Ingress
to enable multiple replicas serving the same HTTPS endpoint
to be able to share the same cert.
Also some minor refactor to allow adding more tests
for cert retrieval logic.
Signed-off-by: Irbe Krumina <irbe@tailscale.com>