Allow templating metadata for multiple environments

[BenoitRanque]

Is your proposal related to a problem?

Currently metadata is a singleton and always replaced in its entirety.

This can be an issue when deployed to multiple environments that have slightly different characteristics, for example:

• API Limits don't make sense in a dev environment
• Read replica configuration may vary across environments dev/stage/prod
• Allow lists again may not make sense in a dev environment
• Users may want to keep multiple query collections as allow lists in prod, but only develop a single list in dev

Describe the solution you'd like

Users should be able to template metadata, or otherwise override certain aspects of the metadata on a per-environment basis.

Users should be able to export and version control overrides separately from the main metadata

Describe alternatives you've considered

This is currently possible if implemented entirely by the user.

Users can also manage this manually, and some metadata exclusive to PRO/Cloud instances will be ignored by OSS instances even if present.

Related issues

#8829