pid helpers support

[jma]

It would be great to support the pid helper:

    'doc_files': {
        'route': '/documents/<pid(doc, record_class=DocRecord):pid_value>/files/<filename>',
    },

This will allows to be consistent with the invenio-records-rest modules. I'm facing some problems when I tried to put some files permissions.

[lnielsen]

Thanks for the suggestion. We're not realisticly going to time to add this, but I'll happily merge PR if you create one. Just ping me on Discord as I might not see the PR due to notification overload.