feat: add optional sidecar files for metadata

[benmcclelland]

This adds the option to store metadata for objects and buckets within a hidden folder alongside the bucket or object: bucket: .vgw_meta. object: .vgw_meta.

Example invocation: ./versitygw -a myaccess -s mysecret posix --sidecar /tmp/gw

The attributes are stored by name within the hidden directory.