Adds startupProbe to prometheus-adapter (#2029)

Issue: https://issues.redhat.com/browse/OCPBUGS-7694



Problem: in clusters with a large nb of CRDs deployed prom-adapter takes too long to discover all of them which makes it fail the livenessProbe

Solution: introduce a startupProbe that gives 3 minutes for prom-adapter to initialize

Signed-off-by: JoaoBraveCoding <jmarcal@redhat.com>