fn framework: bugfix for schema providers + configurable template extensions

[KnVerey]

A small enhancement and a bug fix for the functions framework, in separate commits:

• Configurable extensions for template parser: the default extension we expect is .template.yaml, which is non-standard. This allows one or more other extensions to be explicitly targeted.
• Regression test and fix for APIs that satisfy ValidationSchemaProvider but not Validator: Before this fix, if you implemented ValidationSchemaProvider but not Validator, your schema validation errors would be ignored, leading to the strange behaviour that implementing Validator and returning no errors would cause the incorrectly ignored schema errors to surface.

[k8s-ci-robot]

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[natasha41575]

/lgtm
/approve

[natasha41575]

I'm wondering if we should disable the error wrap lint check, is it important that we ensure all errors are wrapped correctly?

Edit: I'm happy with the change you just pushed!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: KnVerey, natasha41575

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[KnVerey]

I'm wondering if we should disable the error wrap lint check, is it important that we ensure all errors are wrapped correctly?

The linter doesn't complain about all errors--in this case, it is complaining that I'm not wrapping an error from an external package, which seems like a reasonable policy to have to help with debugging. Here's an explanation of when that linter will complain: https://blog.tomarrell.com/post/introducing_wrapcheck_linter_for_go

[natasha41575]

/lgtm