Rework client authentication in SkeletonValidator for clarity

SkeletonValidator was seemingly written to not support public clients at
all. Its authenticate_client_id() explicitly returned `False`, rather than
`pass`-ing like the other methods, and client_authentication_required()
was missing entirely (the default implementation always returns `True`).

This opinionated approach is confusing, especially when writing an
implementation that allows public clients.

The comment on the authenticate_client_id() method is particularly
confusing. Unlike the comments on other methods, which explain the method,
it explains the implementation (returning `False`). As a result, it appears
to say the method should return `False` for public clients, when it should
actually return `False` for confidential clients (and `True` for valid
public clients).

To reduce this confusion, include a client_authentication_required() stub,
`pass` rather than returning `False` in authenticate_client_id(), and
update its comment to describe the method.

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