[Backend] New filesystem backend

[andrewheberle]

This is a new backend that uses the filesystem as a KV store.

It adds a new config option max_file_size to limit the size of files it will read to avoid unbounded reads.

This defaults to 16KB, which may still be too high.

The value of the key is the content of the file, with leading and trailing whitespace removed.

Although not implemented, this probably should have a "prefix" value so that keys like "/myapp/database/url" could be relative to the "prefix" eg:

Prefix = /srv/config
Key = /myapp/database/url
Actual File Path = /srv/config/myapp/database/url