Add the ability to validate extension configuration toml files.

An example file looks something like this:

```toml
[extension]
supported-versions = [
"v1"
]
default-version = "v1"

[v1]
[v1.validation.cross-validates]

[v1.templating]
helpers = []

[v1.generation.requires]
```

Each extension should have its own configuration file describing the exposed API. We should add the ability to run a config validator in tests.

Start by locating the extension configuration handling and the existing test setup. Use the example TOML in the issue to determine the expected configuration shape, then make the tests run a validator against each extension configuration and confirm invalid configurations are rejected.

Written by the indexing model from the issue text.