feat: add samples for Datastore Admin

[jdpedrie]

These tests take ~15 minutes to execute.

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 7 region tags.

• datastoreadmin/quickstart.php:18, tag datastore_admin_client_create
• datastoreadmin/src/entities_export.php:26, tag datastore_admin_entities_export
• datastoreadmin/src/entities_import.php:26, tag datastore_admin_entities_import
• datastoreadmin/src/index_create.php:26, tag datastore_admin_index_create
• datastoreadmin/src/index_delete.php:26, tag datastore_admin_index_delete
• datastoreadmin/src/index_get.php:26, tag datastore_admin_index_get
• datastoreadmin/src/index_list.php:26, tag datastore_admin_index_list

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[jdpedrie]

@bshaffer these tests are reliably passing locally against multiple projects, and failing reliably on kokoro against multiple projects. Is there perhaps something I'm missing with the configuration of the kokoro projects?

[bshaffer]

I believe the cause of the problem is when tested on projects using "Datastore Legacy", the operation to create the index pass, but when tested on projects using "Firestore in Datastore Mode", the tests fail.