Fix MSC4108 'rendez-vous' responses with some reverse proxy in the front of Synapse

[sandhose]

MSC4108 relies on ETag to determine if something has changed on the rendez-vous channel.
Strong and correct ETag comparison works if the response body is bit-for-bit identical, which isn't the case if a proxy in the middle compresses the response on the fly.

This adds a no-transform directive to the Cache-Control header, which tells proxies not to transform the response body.

Additionally, some proxies (nginx) will switch to Transfer-Encoding: chunked if it doesn't know the Content-Length of the response, and 'weakening' the ETag if that's the case. I've added Content-Length headers to all responses, to hopefully solve that.

This basically fixes QR-code login when nginx or cloudflare is involved, with gzip/zstd/deflate compression enabled.