Add tests or linting to ensure extensions register all the providers they claim in extensions.yaml

[tg-msft]

We should add a test that loads every extension and validates that all of its providers match what's declared in extensions.yaml.

[richardpark-msft]

So I just added in a bunch of validation code for the extension registry CI, but it was in JS. I was actually wondering if it'd be better for me to move it into Go, so I could leverage any code in azd for the check.

@JeffreyCA , if you end up setting something like that up for this task (CI job calling into Go code for validating the YAML), let me know and I can port my stuff over too.