Create a URN convention for cross-language transforms

Transform authors have to define URNs when defining new cross-language transforms.

See here for documentation related to this: https://beam.apache.org/documentation/programming-guide/#create-x-lang-transforms

Some example URNs used today.

Kafka read: "beam:external:java:kafka:read:v1"

Kafka write: "beam:external:java:kafka:write:v1"

SQL: "beam:external:java:sql:v1"

Currently this URN is defined by the cross-language transform author (or whoever coverts a given transform to a cross-language transform). They can choose arbitrary URNs.

This is OK as long as we only have few cross-language transforms and as long as we do not deploy transforms with conflicting URNs in the same expansion service but as usage of cross-language transforms increase we could run into conflicts due to this.

We should define a convention so that a cross-language transform author can define a URN that will not run into URNs selected by other cross-language transform authors.