Support enabling DoH for browsers

[Seirdy]

Feature Request Summary

Support enabling browser built-in DNS-over-HTTPS resolvers for a test.

What's the motivation or use case for the change/feature?

Browsers like Firefox and Chromium can query HTTPS DNS resource records for Encrypted Client Hello and for ALPN-like HTTP/3 and HTTP/2 detection. The latter is very relevant for performance testing, as it's the only way to establish an HTTP/3 connection on the initial request. Otherwise, pages are initially fetched over HTTP/2 or lower before HTTP/3 is discovered; switching to HTTP/3 then carries a performance penalty (for example, we lose dynamic HPACK HTTP header compression dictionaries when switching to QPACK).

Firefox (and Chromium? I'm not sure about the latter) require DNS-over-HTTPS to query HTTPS records, perhaps to ensure the integrity of Encrypted Client Hello. Without DoH, it's therefore impossible to test if HTTP/3 on the initial connection works properly.