feat: Adds examples and fixes bugs in spark connector

[jiangmichaellll]

This adds a simple read example and fixes the following bugs:

1. microbatchinputpartition subscriber should initially seek to startoffset instead of endoffset
2. lambda introduced in previous PRs brings this(PslXXXReader) as part of serialization, this PR detached this in lambda.

[codecov]

Codecov Report

Merging #456 (768e61a) into master (48a19d7) will decrease coverage by 0.13%.
The diff coverage is 40.00%.

@@             Coverage Diff              @@
##             master     #456      +/-   ##
============================================
- Coverage     71.27%   71.14%   -0.14%     
  Complexity      918      918              
============================================
  Files           168      168              
  Lines          4864     4873       +9     
  Branches        243      244       +1     
============================================
  Hits           3467     3467              
- Misses         1259     1268       +9     
  Partials        138      138              

Impacted Files	Coverage Δ	Complexity Δ
...le/cloud/pubsublite/spark/PslContinuousReader.java	50.00% <0.00%> (-4.06%)	8.00 <0.00> (ø)
.../pubsublite/spark/PslMicroBatchInputPartition.java	35.00% <16.66%> (-5.00%)	1.00 <0.00> (ø)
...le/cloud/pubsublite/spark/PslMicroBatchReader.java	80.95% <91.66%> (+2.90%)	12.00 <3.00> (ø)
...ubsublite/internal/BlockingPullSubscriberImpl.java	87.93% <0.00%> (-5.18%)	11.00% <0.00%> (-1.00%)

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