Adding new fetch function

[ernestob]

This fetch function retrieves the related objects matching specific meta data.

Usage example:

{def $illustrations = fetch( 'mugoobjectrelations', 'filtered_relations', hash(
    'attribute', $node.data_map.media,
    'filter_by', hash( 'extra_fields', hash( 'meta_data_1', 'illustration' ) ),
) )}

[pkamps]

Looks good. This pull request has originally been created in context of the CSM project. It's running in production there.