feat(spanner): [DO NOT MERGE] add custom replication topology samples

[rahul2393]

Description

Added samples for supporting customer managed instance configurations

Checklist

• [x] Tests pass
• [x] Lint pass: bundle exec rubocop
• [ ] Please merge this PR for me once it is approved.

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 4 region tags.

• spanner/spanner_custom_instance_config.rb:16, tag spanner_create_instance_config
• spanner/spanner_custom_instance_config.rb:73, tag spanner_update_instance_config
• spanner/spanner_custom_instance_config.rb:115, tag spanner_delete_instance_config
• spanner/spanner_custom_instance_config.rb:136, tag spanner_list_instance_config_operations

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[bajajneha27]

Hi @rahul2393 Do we still want to continue working on this?

[rahul2393]

@bajajneha27 closing this PR, once the protos are available I will rebase and create a new PR and link this one for reference.