include SignatureValidation filter with FileBackedHTTPMetadataProvider

[Tom Scavo]

Thanks all. I appreciate your patience and feedback. Since there's no
support for my (evidently whacky) idea, I should just drop this.
Before I do, I'll pose one further question along these lines:

I'm not clear why there are two HTTPMetadataProviders. If the only
difference between the two is the backing file, you only need one
MetadataProvider, right? Is there some historical basis for having
two? If you have any insight, please let me know.

Tom