Support overriding registered functions in configs

[adrianeboyd]

Description

Support overriding registered functions in configs. Previously the registry name was parsed as a section name rather than as a registry name.

Types of change

Bug fix.

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[adrianeboyd]

As reported in: #12618

[adrianeboyd]

This still needs to address the CLI commands that call walk_dict.

[adrianeboyd]

This is getting messy for nested things, let me think about this a bit more...

[adrianeboyd]

So the flag is kind of overkill (and can be renamed if anyone has any better ideas), but if anyone happened to be using walk_dict, then its default behavior doesn't change.

I think since overrides are applied before the config is resolved, it shouldn't be a problem to treat anything at or below the first registered function as a value, even if it contains nested registered functions.