This makes the Spark UI more developer-friendly when analyzing
Overwatch runs.

Job group IDs have the form <workspace name>:<OW module name>

Any use of `.transform( df => df)` may be replaced with
`.transformWithDescription( nt)` after instantiating a `val nt =
NamedTransformation( df => df)` as its argument.

This commit contains one such application of the new extension method.
(See `val jobRunsAppendClusterName` in `WorkflowsTransforms.scala`.)

Some logic in `GoldTransforms` falls through to elements of the
special job-run-action form of Job Group IDs emitted by the platform
but the impact is minimal relative to the benefit to Overwatch
development and troubleshooting.  Even so this form of Job Group ID is
still present in initial Spark events before OW ETL modules begin to
execute.

for greater visibility in Spark UI. `NamedTransformation` type name
now appears in labels' second position.

(cherry picked from commit 2ead752)

TODO: enumerate the regressions this would introduce when the labels set by then platform are replaced this way.