Interim response handling in intermediaries

Dear working group,

the test matrix at https://cache-tests.fyi/#interim has been extended to
include information about how intermediaries handle interim responses. It
currently tests the behavior of nginx, Squid, Apache Traffic Server, httpd,
Varnish, caddy, and HAProxy to see if

1) they accept interim responses from the origin,
2) they forward interim responses to the client, and
3) they cache final responses if an interim response was sent before.

To learn more about the individual test setups, you can click on the test
names and a dialog will open showing what responses the origin generates
and what checks the client performs.

The matrix shows that support for interim responses is mixed. Some
intermediaries support them well while others seem to ignore them. As far
as I know, that's in line with the spec, which allows intermediaries to
drop interim responses.

Please let me know if you have any feedback on this, in particular if the
test setups/checks are incorrect or the proxies need additional
configuration to get interim responses working.

Best regards
Marius