Feature/sub exists

[BenFradet]

This is the sister PR to #1216.

I also removed the pubsubOptions param from newChannel since it doesn't seem to be used.

I'm keen on writing unit tests for PubSubAdmin next if possible.

[regadas]

Hi @BenFradet can you keep the expanded version here?

[BenFradet]

sure 👍

[regadas]

Thx! @BenFradet

[codecov]

Codecov Report

Merging #1483 into master will decrease coverage by 10.48%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           master    #1483       +/-   ##
===========================================
- Coverage   78.92%   68.44%   -10.49%     
===========================================
  Files         170      170               
  Lines        5153     5156        +3     
  Branches      317      396       +79     
===========================================
- Hits         4067     3529      -538     
- Misses       1086     1627      +541

Impacted Files	Coverage Δ
...in/scala/com/spotify/scio/pubsub/PubSubAdmin.scala	0% <0%> (ø)	⬆️
.../scala/com/spotify/scio/bigquery/client/Jobs.scala	0% <0%> (-100%)	⬇️
.../spotify/scio/bigquery/BigQueryPartitionUtil.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%)	⬇️
...potify/scio/runners/dataflow/DataflowContext.scala	0% <0%> (-92.86%)	⬇️
...cala/com/spotify/scio/bigquery/client/JobOps.scala	0% <0%> (-90.7%)	⬇️
... and 24 more

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