public-access: add an indicative header for requests that are made by guest users

[peb-adr]

Current behavior
Not the following. (lol)

Wanted behavior
The following:

When a user accesses a meeting publicly (i.e. clicks the "public access" button) all following requests should add an http header indicating they are indeed accessing publicly.
Consequently as soon as the users logs in the header should no longer be set.

Additional context
This will make it easier to separate and prioritize requests and take action on that, e.g. route them differently.

[peb-adr]

Name proposal: X-Public-Access: true

[bastianjoel]

Status is currently set by a cookie which is sufficient for now. Using a header would have the benefit that we could set this just for api requests and not for requests to client assets.