feat(cloud): move reference commands into their product commands

[duboolivier]

Description

Reference data (plans, flavors, engines, versions, images...) was hidden
under cloud reference <product> <cmd>, which is hard to discover. Move
each reference subcommand directly into its product command and remove the
cloud reference command:

- cloud managed-database list-plans / list-node-flavors / list-engines
- cloud managed-analytics list-plans / list-node-flavors / list-engines
- cloud managed-registry list-plans / list-regions
- cloud managed-rancher list-versions / list-plans
- cloud loadbalancer list-flavors / get-flavor
- cloud instance list-flavors / list-images

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (improvement of existing commands)
• Breaking change (fix or feature that can break a current behavior)
• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have commented my code
• I ran go mod tidy
• I have added tests that prove my fix is effective or that my feature works