feat(spanner): add samples for instance partitions

[varuncnaik]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

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Fixes #<issue_number_goes_here> ☕️

If you write sample code, please follow the samples format.

[snippet-bot]

Here is the summary of changes.

You are about to add 1 region tag.

• samples/snippets/src/main/java/com/example/spanner/CreateInstancePartitionSample.java:19, tag spanner_create_instance_partition

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[rahul2393]

@varuncnaik please fix lint,

mvn  com.coveo:fmt-maven-plugin:format