Allow compat session devices to have spaces #4067
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The context for this is that the Matrix spec allows basically anything in the device ID. With MSC3861, we're restricting this to strings that can be represented as scopes. Whilst this works well for next-gen auth sessions, compatibility/legacy sessions still can have characters that can't be encoded (mainly spaces) in them. To work around that, we added in MAS a behaviour where the device_id is given as an explicit property of the token introspection response, and remove it from the scope. Because we don't expect users to rollout new Synapse and MAS versions in sync, we needed a way to 'advertise' support for this behaviour: the easiest way to do that was through an extra header in the introspection response. On the longer term, I expect MAS and Synapse to move away from the introspection endpoint, and instead define a specific API for Synapse -> MAS communication. PR on the MAS side: element-hq/matrix-authentication-service#4067
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The Matrix spec allows anything in the device ID.
With MAS and MSC3861, we restricted that to anything that can be represented as a scope.
But this is valid for next-gen auth sessions, not necessarily for compatibility ones.
Slight problem is, for compatibility sessions, we're still giving out the device ID as a scope to Synapse, so that won't fly with a device ID that can't be represented that way.
To sidestep this, we now return the device ID as an explicit property in the introspection response.
This means the homeserver still has to adapt to this behaviour, so we can't just remove the device ID from the scope.
This is why we're introducing a header to the introspectio endpoint which lets the homeserver advertise its support for this behaviour.
Fixes #2724
Fixes #4027
Synapse PR: element-hq/synapse#18174