Add experimental support for MSC4502: Targeted and unrestricted room member queries

[Johennes]

This is another stopgap towards https://github.com/element-hq/voip-internal/issues/641 and adds experimental support for MSC4502.

This adds a new endpoint /rooms/{roomId}/is_joined to query if a user or server is joined to a room known to the homeserver. Access to the endpoint is guarded behind a new OAuth scope that can be assigned when registering application services.

I have split the changes up into three commits to simplify the review.

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