admin: convert filer address to gRPC form before dispatch (#9523)

The master returns each registered filer in pb.ServerAddress dual-port
form (host:httpPort.grpcPort, e.g. 10.0.0.1:8888.18888). The admin's
plugin context builder forwarded that string verbatim as
filer_grpc_address, so workers calling grpc.DialContext on it failed
every job in ~3ms with "dial tcp: lookup tcp/8888.18888: unknown port".

Run each entry through pb.ServerAddress.ToGrpcAddress before populating
ClusterContext.FilerGrpcAddresses.

The lifecycle integration test now pins filer.port.grpc to a value that
breaks the FILER_PORT+10000 assumption, and a new dispatch test drives
the admin's /api/plugin/job-types/s3_lifecycle/run path end-to-end and
asserts the dispatched job both reaches the filer and deletes the
backdated object.

test/s3/lifecycle/s3_lifecycle_test.go

@@ -40,12 +40,14 @@ const (
 	defaultS3Endpoint     = "http://localhost:8333"
 	defaultS3GrpcEndpoint = "localhost:18333"
 	defaultMasterEndpt    = "http://localhost:9333"
-	defaultFilerGRPC      = "localhost:18888"
+	defaultAdminEndpoint  = "http://localhost:23646"
+	// Pinned off the FILER_PORT+10000 convention; see Makefile.
+	defaultFilerGRPC      = "localhost:18890"
 	bucketLifecycleXMLKey = "s3-bucket-lifecycle-configuration-xml"
 	bucketsPath           = "/buckets"
 	accessKey             = "some_access_key1"
-	secretKey           = "some_secret_key1"
-	region              = "us-east-1"
+	secretKey             = "some_secret_key1"
+	region                = "us-east-1"
 )
 
 func envOr(key, def string) string {