Stop rejecting RPPs if unrecognizable absent files are fileListed

Stop rejecting RPPs if unrecognizable absent files are fileListed

RFC 9286:

> The RP MUST acquire all of the files enumerated in the manifest
> (fileList) from the publication point. If there are files listed in
> the manifest that cannot be retrieved from the publication point,
> the RP MUST treat this as a failed fetch.

This was clashing with Fort's default rsync filters because they were
preventing unknown extensions from being downloaded:

> rsync (...) --include=*.cer --include=*.crl --include=*.gbr \
>	--include=*.mft --include=*.roa --exclude=* (...)

Which will be a problem whenever the IETF defines new legal repository
extensions, such as .asa.

Therefore, ignore unknown manifest fileList extensions. This technically
violates RFC 9286, but it's necessary evil given that we can't trust
repositories to always only serve proper RPKI content.

Fixes #155.