add MSC2723 forwarding meta data for more compatibility

[dozro]

Description

adding the forwarding meta data as defined in MSC2723, but only for messages forwarded from a public room and without setting the sender key, as it was a design decision on our previous implementation to not forward sender information

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings