imap-urlauth: Store urlauth keys to the new mailbox attributes.

imap-urlauth: Use mailbox GUID instead of mailbox name for dict keys

Added support for IMAP URLAUTH and URLAUTH=BINARY extensions Extends imap service with URLAUTH and URLAUTH=BINARY support: - Adds new commands URLFETCH, GENURLAUTH and RESETKEY. Creates imap-urlauth service in src/imap-urlauth. Functionality common to both the imap and imap-urlauth services is located in src/lib-imap-urlauth. TODO: - use mailbox GUIDs instead of names - doveadm command to delete stale urlauth entries? - add delay when attempting to access nonexistent user - create urlauth-worker queue, similar to how indexer-worker works (could we share code?..)