Allow customising the <meta name="referrer" /> tag #789

Is your feature request related to a problem? Please describe.
I have a need to restrict public access to an S3 bucket for video objects. The default same-origin value for the referrer in the DefaultDocument component prevents video elements in the studio from sending the origin to allow ACL to kick in

Describe the solution you'd like
It would be great to be able to just configure the referrer meta tag for the studio using the sanity config file.

Describe alternatives you've considered
I have created a custom _document.tsx component in my studio root using the DefaultDocument from the sanity cli.