Allow users to select the cloud region when installing a starter template

[emmenko]

Is your feature request related to a problem? Please describe.

Installing one of the Merchant Center starter templates (customizations) results by default in using the GCP EU environment.

However, for customers using a different CoCo region it might get confusing if they forget to change the cloudIdentifier to the region of their choice.

Relates to #3553

Describe the solution you'd like

• Improve the DX of getting started with MC customizations.
• The create-mc-app CLI should prompt the user to ask about the region of their choice, defaulting to gcp-eu.
  • The option to define the region should also be possible via a CLI option, e.g. --cloud-identifier=gcp-au for programmatic usage.