libgpg-error: add mirror

Closes #19091.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
botan 1.10.5
wip: first pass at fixing validation

 - remove direct ivar accesses
 - remove set_instance_variable
 - remove validate_variable
 - introduce set_spec, validate_attributes, and determine_active_spec
 - add tests for attribute validation