feat: PSL spark sql streaming utility classes

[jiangmichaellll]

Spark sql streaming POJOs and utility classes.

Also making com.google.cloud.pubsublite.Offset serializable, as it's needed for serialization into executor later.

[codecov]

Codecov Report

Merging #391 (cdab064) into master (a15ed0b) will decrease coverage by 1.40%.
The diff coverage is 22.95%.

@@             Coverage Diff              @@
##             master     #391      +/-   ##
============================================
- Coverage     72.39%   70.98%   -1.41%     
- Complexity      783      796      +13     
============================================
  Files           148      154       +6     
  Lines          4191     4312     +121     
  Branches        204      210       +6     
============================================
+ Hits           3034     3061      +27     
- Misses         1042     1134      +92     
- Partials        115      117       +2     

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/pubsublite/spark/Constants.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...cloud/pubsublite/spark/PslCredentialsProvider.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...e/cloud/pubsublite/spark/PslDataSourceOptions.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...m/google/cloud/pubsublite/spark/PslSparkUtils.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...google/cloud/pubsublite/spark/PslSourceOffset.java	77.41% <77.41%> (ø)	11.00 <11.00> (?)
.../main/java/com/google/cloud/pubsublite/Offset.java	100.00% <100.00%> (ø)	3.00 <1.00> (ø)
...gle/cloud/pubsublite/spark/PslPartitionOffset.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
... and 3 more

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