[tor-onions] Announcing Onion-Website with x-onion response-header?

[MacLemon]

Hi!

Just an idea:
What about announcing that your site is also available via onion-service by sending an x-onion HTTP response header on your HTTPS website?

For example:
The clearweb site https://www.torproject.org/ could send a header like this:
x-onion:http://examplefoobarbaz.onion/

Or in case you can actually provide a valid TLS certificate for your Onion:
x-onion:https://examplefoobarbaz.onion/

Another idea would be to also provide the fingerprint of the to-be-expected TLS certificate. This could look like so:

x-onion:cert-sha256="1h89m/yelEy6l1poFiXZQbJ1s6BkrOquBl7Fd+0EOO0="; https://examplefoobarbaz.onion/
Similar to what is done with HPKP headers, but without pinning.

Follow up question:
How could this be done with non-HTTP services? (XMPP, SMTP, etc.)

Best regards
@MacLemon
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