Transform rehydrator registration should be idempotent

Currently, if the same URN is registered twice, it results in an exception. Instead, if it is a duplicate registration (for example, caused by double-shading of the service loader config) it should succeed. This requires probably good equals methods for the rehydrators.

Other minor issues: it will fail at load time and from then throw class not found errors, which is extremely annoying. It should instead load lazily so we can always get a good stack trace.

Imported from Jira [BEAM-3069](https://issues.apache.org/jira/browse/BEAM-3069). Original Jira may contain additional context.
Reported by: kenn.

Start by locating the transform rehydrator registration and the service-loader configuration that supplies it. Reproduce duplicate URN registration and the load-time class failure, then verify that duplicate registrations succeed and failures retain a useful stack trace.

Written by the indexing model from the issue text.