Make happy-eyeballs proxy aware

[mxinden]

Should we make HappyEyeballs proxy aware? E.g. should it know that the proxy is resolving the domain? Should it still trigger an HTTP RR lookup to see whether the remote supports HTTP/3? Should it first do MASQUE connect-udp and HTTP/3 and then HTTP CONNECT with HTTP/2?

[zirngibl]

Could this be extended to make the proxy happy-eyeballs aware, e.g., adding some sort of signal to test specific IP versions?
This is not possible by the client alone but might an interesting dicussion.