Allow fetching of extraMetadata

[timwsuqld]

Feature Request

Q	A
New Feature	yes
BC Break	yes

Scenario / Use-case

Many backends provide more metadata. We currently populate FileAttributes with the data, but don't provide a way to access it. While we can't provide interfaces into any of that data in a way that would allow cross adapter use, we can just provide that data. That way when using the Flysystem, if you either know the specific filesystem adapter that will be used, or can use the metadata with a fallback, you can make for a more efficient system. For example, a few systems return the md5sum in the metadata, this can be used to check if a file matches an existing file before updating the file. It's easy enough to check the returned metadata for that field, and if not present, then code a fallback method (it may be more costly to replace the file if it hasn't changed).

I believe this would be a BC due to the addition of methods to the interfaces.

Summary

Expose the extraMetadata attribute to the end user.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[PaolaRuby]

this would have been nice in v3

[neffets]

I have use of this feature too.
I want provide further meta-data for attribution and licenses in the extraMetadata

[marden]

It's definitely a high-value feature, especially for S3 as it holds a lot of useful metadata.