feat: PSL Connector Writer support

[jiangmichaellll]

No description provided.

[codecov]

Codecov Report

Merging #121 (165ee80) into master (20f3366) will increase coverage by 0.55%.
The diff coverage is 54.58%.

@@             Coverage Diff              @@
##             master     #121      +/-   ##
============================================
+ Coverage     58.96%   59.52%   +0.55%     
- Complexity       92      129      +37     
============================================
  Files            18       24       +6     
  Lines           580      751     +171     
  Branches         24       31       +7     
============================================
+ Hits            342      447     +105     
- Misses          232      296      +64     
- Partials          6        8       +2     

Impacted Files	Coverage Δ	Complexity Δ
...le/cloud/pubsublite/spark/PslContinuousReader.java	58.53% <ø> (ø)	8.00 <0.00> (ø)
...m/google/cloud/pubsublite/spark/PslDataSource.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...le/cloud/pubsublite/spark/PslMicroBatchReader.java	86.27% <ø> (ø)	11.00 <0.00> (ø)
...ite/spark/internal/CachedPartitionCountReader.java	0.00% <ø> (ø)	0.00 <0.00> (?)
...blite/spark/internal/LimitingHeadOffsetReader.java	70.58% <ø> (ø)	5.00 <0.00> (?)
...te/spark/internal/MultiPartitionCommitterImpl.java	83.33% <ø> (ø)	14.00 <0.00> (?)
...sublite/spark/internal/PslCredentialsProvider.java	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...ud/pubsublite/spark/PslWriteDataSourceOptions.java	14.89% <14.89%> (ø)	3.00 <3.00> (?)
...ud/pubsublite/spark/internal/CachedPublishers.java	18.75% <18.75%> (ø)	2.00 <2.00> (?)
...oud/pubsublite/spark/PslReadDataSourceOptions.java	19.56% <33.33%> (ø)	3.00 <2.00> (?)
... and 12 more

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[dpcollins-google]

LGTM after remaining fixes, ping me after those are done.

[dpcollins-google]

clirr is going to be mad about this PR, you'll need to add allowed exceptions for all the things you've moved to an internal file.

Also, look at the clirr file for the main library and allow breaking changes in all internal directories.