feat(Spanner): Add sample for add and drop database roles for Spanner

[ajupazhamayil]

This PR creates a sample for Spanner database role add and drop.

[snippet-bot[bot]]

Here is the summary of changes.

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• spanner/src/add_drop_database_role.php:26, tag spanner_add_and_drop_database_role

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[ajupazhamayil]

rerun tests please

[ajupazhamayil]

kokoro:force-run

[vishwarajanand]

Since this sample is not yet published, I am okay with merging this change.