include SignatureValidation filter with FileBackedHTTPMetadataProvider

[Cantor, Scott]

> 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.

I alluded to that earlier. Other than code factoring I don't know why two types are needed for any normal deployment. I suppose there could be embedded scenarios where that code might be useful to somebody that couldn't back the file up.

-- Scott