Add method to access hatrac metadata

[carlkesselman]

There is currently no way within the library to access the metadata associated with a hatrac URL. It would be good to provide this functionality. In addition, get/put methods should resturn metadata as well.

[karlcz]

We need to think about what we want here. The metadata available to regular clients is just what you can get from a HEAD request on the object path, inspecting the response.headers dictionary.

An owner of the object can also manipulate the path;metadata and path;metadata/key sub-resources which allow some management of this state. However, these will often respond with Forbidden errors for less privileged clients.

[karlcz]

Also, I'm not sure what you mean by getting metadata from a PUT request. The only new information produced by PUT should be the versioned path, which is already returned. Shouldn't everything else be known by the client already? E.g. file size and hash?

[carlkesselman]

Not sure. Perhaps issue is just documentation. Was not apparent at first blush that while using deriva-py I would use the head request to get the metadata like MD5 code and length as returned by server. I was thinking that as an alternative that information would be returned by default along with the path.