Make FilenameProvider optional on PathProvider impl

[DiamondJoseph]

Closes #546

Required for Diamond expectation of file names.

[coretl]

I think there are 2 use cases we need to support:

1. Where Directory and Filename can be mixed together in different combinations, e.g. static directory + datestamp filename or static directory + uuid filename
2. Where Directory and Filename are so site specific they will never be mixed together with other providers

To support 2 I think we need the option for a PathProvider that has no FilenameProvider, so it can make the whole path itself. For 1 then I think we need specific PathProviders (like StaticPathProvider) to support taking a FilenameProvider to delegate to

I think we can support all this by removing _filename_provider from the PathProvider protocol

[DiamondJoseph]

After discussion IRL, we've opted to do both: remove the requirement for the PathProvider to have a FilenameProvider, but also adjust the signature of the FilenameProvider to match the PathProvider. This can propagate from the PathProvider call down.