Load v2 GroupMetadata attributes

[dominikl]

• Load the attributes of the v2 GroupMetadata.
• Make them available on the GroupMetadata interface level via attributes()
• Also make the Groupmetadata availabe via Group.metadata()

To test:

String path = "https://uk1s3.embassy.ebi.ac.uk/idr/zarr/v0.4/idr0073A/";
String zarr = "9798462.zarr";
StoreHandle handle = new HttpStore(path).resolve(zarr);
Group group = Group.open(handle);
System.out.println(group.metadata().attributes());

(address issue #23 (comment) )

[dominikl]

@brokkoli71 Please don't hesitate to just close this PR in case you're already working on that too! :-)

[brokkoli71]

I wasn't. Thanks for adding it! :)

[brokkoli71]

Thanks for the addition! In case you're not currently working on this, I'll merge it, apply the requested changes there, and add some tests.

[dominikl]

Thank you very much. Yes, sorry, haven't got round to add test for this.