Include config_declaration.yaml into MANIFEST

[pdelboca]

We need to include the new config_declaration.yaml file into the MANIFEST file.

    return wrapper(plugin)
  File "/usr/lib/ckan/venv/lib/python3.8/site-packages/ckan/plugins/blanket.py", line 443, in wrapper
    key.implement(plugin, subject)
  File "/usr/lib/ckan/venv/lib/python3.8/site-packages/ckan/plugins/blanket.py", line 198, in implement
    subject = self.get_subject(plugin)
  File "/usr/lib/ckan/venv/lib/python3.8/site-packages/ckan/plugins/blanket.py", line 177, in get_subject
    return _mapping[self].extract_subject(plugin)
  File "/usr/lib/ckan/venv/lib/python3.8/site-packages/ckan/plugins/blanket.py", line 239, in _declaration_file_extractor
    raise FileNotFoundError("config_declaration.EXT")
FileNotFoundError: config_declaration.EXT