feat(spanner): add samples for fine grained access control

[rahul2393]

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Fixes #<issue_number_goes_here> ☕️

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

EnableFineGrainedAccess sample requires service account to work, so skipping integration tests for the same.

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/snippets/src/main/java/com/example/spanner/AddAndDropDatabaseRole.java:19, tag spanner_add_and_drop_database_role
• samples/snippets/src/main/java/com/example/spanner/EnableFineGrainedAccess.java:19, tag spanner_enable_fine_grained_access
• samples/snippets/src/main/java/com/example/spanner/ListDatabaseRoles.java:19, tag spanner_list_database_roles
• samples/snippets/src/main/java/com/example/spanner/ReadDataWithDatabaseRole.java:19, tag spanner_read_data_with_database_role

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[rajatbhatta]

@rahul2393: I think we should add integration tests for the samples similar to https://github.com/googleapis/java-spanner/pull/2007/files#diff-cbbf8310726971f9d351d2727d761558fcea52286bd9b86628e83e763f8b63f0. Please add @RunWith(JUnit4.class) to the class.

[rajatbhatta]

Lgtm in general, just a couple of comments. You can merge it once the tests pass post those changes.