docs: Samples and tests for Instance Admin Client APIs

[alkatrivedi]

This PR contains sample files and their integration tests for the instance admin client APIs.

[snippet-bot]

Here is the summary of changes.

You are about to add 8 region tags.

• samples/v2/get-instance-config.js:23, tag spanner_get_instance_config
• samples/v2/instance-config-create.js:27, tag spanner_create_instance_config
• samples/v2/instance-config-delete.js:26, tag spanner_delete_instance_config
• samples/v2/instance-config-get-operations.js:23, tag spanner_list_instance_config_operations
• samples/v2/instance-config-update.js:26, tag spanner_update_instance_config
• samples/v2/instance-with-processing-units.js:19, tag spanner_create_instance_with_processing_units
• samples/v2/instance.js:20, tag spanner_create_instance
• samples/v2/list-instance-configs.js:23, tag spanner_list_instance_configs

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