add filters and priority

lib/gat/gatcaddyfile/discoverer.go

lib/gat/handlers/discovery/cluster.go

...gat/handlers/discovery/discoverers/digitalocean/config.go

...handlers/discovery/discoverers/digitalocean/discoverer.go

...lers/discovery/discoverers/google_cloud_sql/discoverer.go

...lers/discovery/discoverers/zalando_operator/discoverer.go

lib/gat/handlers/discovery/module.go

lib/gat/handlers/pool/recipe.go

lib/gat/handlers/pool/spool/pool.go

lib/util/strutil/matcher.go

lib/util/strutil/matcher_test.go