Require children to be registered in service registry when a service registry is specified

[cortadocodes]

Feature request

Use Case

If one or more service registries are specified in octue.yaml or the ask method, only service revisions in the registries should be addressable. An error should be raised otherwise. A keyword argument could be added to allow explicit overriding of this behaviour.

Current state

The service registry is only checked when a revision tag isn't provided, allowing unregistered service revisions to be addressed.

[cortadocodes]

@thclark do we want the behaviour when no service registry is provided to be:

• don't validate children against any registry, or
• attempt to validate children against the official Octue registry

We haven't yet created our registry but it wouldn't be a massive amount of work. I'll begin on option a) for now