lib-storage: Add support for "validated attributes"

lib-storage: Add support for "validated attributes"

If an attribute is marked with MAIL_ATTRIBUTE_INTERNAL_FLAG_VALIDATED, it's
assumed to be "validated". This means that it has a set() callback that
validates the value, which at minimum means that its size isn't excessively
large.

MAIL_ATTRIBUTE_TYPE_FLAG_VALIDATED can be used with set/get/iterate to allow
access only to these validated attributes. Trying to access non-validated
attributes will result in error.