Add .spec.clusterSpecList field to MongoDB

[m1kola]

Summary

Add .spec.clusterSpecList field to MongoDB. This will enable us to make MongoDB to be capable once we add controller handling of the new fields.

Proof of Work

N/A - just a new field at the moment.

Checklist

• Have you linked a jira ticket and/or is the ticket in the title?
• Have you checked whether your jira ticket required DOCSP changes?
• Have you added changelog file?
  • use skip-changelog label if not needed
  • refer to Changelog files and Release Notes section in CONTRIBUTING.md for more details