fix: azure backend errors with storage url and container metadata

The container URL was adding an extra / when the service URL ended
with /. This results in an invalid container URI error from Azure.

This also fixes the container metadata keys to not include "-" that
the Azure service disallows. The metadata values are base64 encoded
to prevent any special chacter handling on Azure service side.

The content type is set for uploads consistent with S3 expectations.

Fixes #736