Add support for String/ByteString TF Bucket key types

[nmaraston]

String/ByteString key types are supported when writing TF Examples to SMB buckets (since the type need only be encodeable). These types are however not supported when reading TF Examples from said buckets.

This PR adds support for String/ByteString key types when extracting SMB bucket keys.

[martinbomio]

lgtm!

[codecov]

Codecov Report

Merging #3098 into master will decrease coverage by 2.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3098      +/-   ##
==========================================
- Coverage   72.20%   69.65%   -2.56%     
==========================================
  Files         232      232              
  Lines        7697     7697              
  Branches      352      333      -19     
==========================================
- Hits         5558     5361     -197     
- Misses       2139     2336     +197     

Impacted Files	Coverage Δ
...scala/com/spotify/scio/bigquery/client/Cache.scala	0.00% <0.00%> (-74.08%)	⬇️
.../spotify/scio/bigquery/client/BigQueryConfig.scala	0.00% <0.00%> (-70.59%)	⬇️
.../spotify/scio/bigquery/BigQueryPartitionUtil.scala	0.00% <0.00%> (-52.50%)	⬇️
...scala/com/spotify/scio/bigquery/BigQueryUtil.scala	50.00% <0.00%> (-50.00%)	⬇️
...otify/scio/bigquery/syntax/ScioContextSyntax.scala	19.35% <0.00%> (-41.94%)	⬇️
...la/com/spotify/scio/bigquery/client/QueryOps.scala	0.73% <0.00%> (-39.71%)	⬇️
.../scala/com/spotify/scio/bigquery/StorageUtil.scala	0.00% <0.00%> (-36.85%)	⬇️
...com/spotify/scio/bigquery/types/BigQueryType.scala	58.33% <0.00%> (-29.17%)	⬇️
...la/com/spotify/scio/bigquery/client/TableOps.scala	0.00% <0.00%> (-25.00%)	⬇️
...com/spotify/scio/bigquery/types/TypeProvider.scala	52.11% <0.00%> (-22.54%)	⬇️
... and 2 more

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