Add recursive validation to config

[mattsains]

Closes #4584

Description:
This PR, which is heavily based on @FreakyNobleGas 's work, allows for children of config structs to define their own Validate method. All nodes' Validate methods will be checked instead of just the root of the config element.

If a validation error is found within the config tree, it will be surfaced with a message like this:

extension "ExtensionName" has invalid configuration in "ChildList[1].AnElementWithValidator": The error message returned by the validator within AnElementWithValidator

Link to tracking Issue:
#4584

Testing:
I added two new tests to check that children of config are validated. I ran these tests through make all in the root of the project.

I think the testing I've done is a bit weak just because of my unfamiliarity with the project, so I'm happy to do more testing if needed.

Open question: If a config struct has a Validate method implemented, but that validation succeeds, should we continue checking inside the children of that object, or should we consider the entire object "checked" by that implementation? Right now, I've chosen the former option.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: mattsains / name: Matthew Sainsbury (cacd895)

[bogdandrutu]

Please take a look at #6545, where we want to add the validation.

[mattsains]

Damn, that sucks, looks like my PR is useless then (I'll close it). @bogdandrutu is any help needed on the other PR? Otherwise I'll just find something else to do