Allow adding attributes to the root XML node

Problem/Motivation

See: #2886357-66: Changing default "" and "" tags in XML export → for more details.

This doesn't need to be done as a single text field, as the Symfony serializer actually has built-in support for doing these already, so we could provide nice escaping etc. and not do broken plain string swaps.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes