Allow custom transform names for union all

[anish749]

The transform name was hard coded to UnionAll for the union all transform, and doesn't pick up the call site name as the transform name. Adding unique names / user defined custom names, helps in easy tracking in the DataFlow UI.

[codecov]

Codecov Report

Merging #1499 into master will decrease coverage by 10.7%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master    #1499       +/-   ##
===========================================
- Coverage   79.16%   68.45%   -10.71%     
===========================================
  Files         173      173               
  Lines        5250     5250               
  Branches      312      315        +3     
===========================================
- Hits         4156     3594      -562     
- Misses       1094     1656      +562

Impacted Files	Coverage Δ
.../src/main/scala/com/spotify/scio/ScioContext.scala	85.83% <100%> (-1.57%)	⬇️
...in/scala/com/spotify/scio/values/SCollection.scala	91.41% <100%> (-0.09%)	⬇️
.../scala/com/spotify/scio/bigquery/client/Jobs.scala	0% <0%> (-100%)	⬇️
.../spotify/scio/bigquery/BigQueryPartitionUtil.scala	0% <0%> (-100%)	⬇️
...cala/com/spotify/scio/spanner/client/Spanner.scala	0% <0%> (-100%)	⬇️
...ala/com/spotify/scio/bigquery/client/LoadOps.scala	0% <0%> (-100%)	⬇️
.../com/spotify/scio/bigquery/client/ExtractOps.scala	0% <0%> (-100%)	⬇️
...scala/com/spotify/scio/bigquery/MockBigQuery.scala	0% <0%> (-94.74%)	⬇️
...la/com/spotify/scio/bigquery/dynamic/package.scala	0% <0%> (-94.12%)	⬇️
...scala/com/spotify/scio/bigquery/client/Cache.scala	0% <0%> (-94.12%)	⬇️
... and 28 more

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[regadas]

Thx! 👍