The core.Challenge.ProvidedKeyAuthorization field is problematic, both
because it is poorly named (which is admittedly easily fixable) and
because it is a field which we never expose to the client yet it is held
on a core type. Deprecate this field, and replace it with a new
vapb.PerformValidationRequest.ExpectedKeyAuthorization field.
Within the VA, this also simplifies the primary logic methods to just
take the expected key authorization, rather than taking a whole (largely
unnecessary) challenge object. This has large but wholly mechanical
knock-on effects on the unit tests.
While we're here, improve the documentation on core.Challenge itself,
and remove Challenge.URI, which was deprecated long ago and is wholly
unused.
Part of https://github.com/letsencrypt/boulder/issues/7514
09693f03dc