feat: [DO NOT MERGE] support customer managed instance configurations

[olavloite]

DO NOT MERGE: Depends on changes in the client library that have not yet been released.

Adds samples for working with user managed instance configurations for Cloud Spanner.

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 4 region tags.

• spanner/src/create_instance_config.php:26, tag spanner_create_instance_config
• spanner/src/delete_instance_config.php:26, tag spanner_delete_instance_config
• spanner/src/list_instance_config_operations.php:26, tag spanner_list_instance_config_operations
• spanner/src/update_instance_config.php:26, tag spanner_update_instance_config

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[olavloite]

cc @rahul2393

[rahul2393]

Closing this one, cherry picked here https://github.com/GoogleCloudPlatform/php-docs-samples/pull/1696