mail-lua: Implement mailbox metadata API

mail-lua: Implement mailbox metadata API

Adds methods for accessing and manipulating mailbox metadata

 * user:metadata_get("key", "key", ..) - returns value for keys
 * user:metadata_set("key", "value") - sets value for key
 * user:metadata_unset("key", "value") - unsets key
 * user:metadata_list("prefix", "prefix", ...) - lists keys in dict for prefixes

 * mailbox:metadata_get("key", "key", ..) - returns value for keys
 * mailbox:metadata_set("key", "value") - sets value for key
 * mailbox:metadata_unset("key", "value") - unsets key
 * mailbox:metadata_list("prefix", "prefix", ...) - lists keys in dict for prefixes

User metadata is stored under /(private|shared)/vendor/vendor.dovecot/pvt/server to
mailbox INBOX, and is accessible with mailbox metadata methods with INBOX.

Also mail lua exports constants to simplify usage

dovecot.storage.MAILBOX_ATTRIBUTE_PREFIX_DOVECOT
"vendor/vendor.dovecot/"
dovecot.storage.MAILBOX_ATTRIBUTE_PREFIX_DOVECOT_PVT
"vendor/vendor.dovecot/pvt/"
dovecot.storage.MAILBOX_ATTRIBUTE_PREFIX_DOVECOT_PVT_SERVER
"vendor/vendor.dovecot/pvt/server/"