feat: Close clients gracefully

[jiangmichaellll]

This would help avoiding https://screenshot.googleplex.com/9fxT2VgyZWWptdA similar to:

ManagedChannelOrphanWrapper: *~*~*~ Channel ManagedChannelImpl{logId=3, target=us-central1-pubsublite.googleapis.com:443} was not shutdown properly!!! ~*~*~*

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• samples/snapshot/pom.xml
• samples/snippets/pom.xml

[snippet-bot]

Here is the summary of changes.

You deleted 2 region tags.

• samples/snippets/pom.xml:28, tag pubsublite-spark_install_without_bom
• samples/snippets/pom.xml:29, tag pubsublite-spark_java_dependencies

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[codecov]

Codecov Report

Merging #56 (4547de2) into master (2bbd9df) will increase coverage by 0.11%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master      #56      +/-   ##
============================================
+ Coverage     58.28%   58.39%   +0.11%     
  Complexity       81       81              
============================================
  Files            17       17              
  Lines           525      524       -1     
  Branches         18       18              
============================================
  Hits            306      306              
+ Misses          216      215       -1     
  Partials          3        3              

Impacted Files	Coverage Δ	Complexity Δ
...le/cloud/pubsublite/spark/PslContinuousReader.java	58.97% <0.00%> (ø)	7.00 <0.00> (ø)
...m/google/cloud/pubsublite/spark/PslDataSource.java	0.00% <0.00%> (ø)	0.00 <0.00> (ø)
...blite/spark/PslMicroBatchInputPartitionReader.java	63.88% <0.00%> (+4.91%)	7.00 <0.00> (ø)
...le/cloud/pubsublite/spark/PslMicroBatchReader.java	86.00% <0.00%> (-3.59%)	11.00 <0.00> (ø)

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