[Feature Request]: Add generic metadata sync

[sboesen]

What feature are you suggesting?

Related to #64 - Alibaba Cloud doesn't support standard S3 metadata so we should store modification time in the plugin metadata file.

What remote cloud services are you using, or suggesting adding the feature to?

S3, OneDrive for personal, Dropbox, webdav, others

Ensure no sensitive information

• I ensure that no sensitive information is submitted in the issue.

[kadisonm]

This might be good in conjunction with #67 and storing data on files with their name, modification date, path and if they're empty so empty files can be created again on sync.

I'm curious if that could speed up sync times slightly for other providers since empty files wouldn't need to be uploaded.

[sboesen]

Tracking work in #97