Go code should get standard URNs from proto annotations

https://github.com/apache/beam/pull/4672 makes standard URNs of various Beam entities be defined in annotations on some enums in the protos. Java and Python now don't hard-code the URNs, instead they get them from the proto annotations.

However, Go still hard-codes the URNs. This JIRA is to make Go do something like what Java and Python do.

(I tried coding this up, but couldn't come up with a good Go API for this - the Java and Python ways don't translate into idiomatic Go as far as I can tell; it's also complicated by the fact that Go protobuf compiler currently doesn't give easy access to enum descriptors: https://github.com/golang/protobuf/issues/364)

Imported from Jira [BEAM-4179](https://issues.apache.org/jira/browse/BEAM-4179). Original Jira may contain additional context.
Reported by: jkff.