Respect and support service extensions in environment overrides

[ezodude]

Resolves: #361

This PR implements the following extension strategy:

• Service config extensions can be added to the compose sources.
• Service config extensions can be added to the environment override files.
• Service config extensions will always be merged back to the sources config.
  • Pre merge: If an extension does not exist in the source ... Post merge: it will be added to the source.
  • Pre merge: If an extension exists in the source ... Post merge: it will be merged in the source, with the environment
    override extension entries overriding the source's entries.
• Extensions are not reconciled.

This strategy allows us to add extensions to environment overrides, that will persist even if they don't exist in the original compose sources. And, merge in overrides if compose sources already contain the extension.

[marcinc]

LG. Worth adding a spec for case where extensions are only present in the override?