Async certificate resolver

[MatthewCash]

Is there a way to asynchronously load and use a certificate after ClientHello? This is possible in tokio-rustls with LazyConfigAcceptor but I have not found a way to do it with qinn.

My use case is for the server to dynamically pick a cert based on SNI, read the cert's file asynchronously, and then use it for the connection.

[djc]

Yeah, we don't have support for the Acceptor yet. I think there was some recent discussion (probably in Discord), but it's a little gnarly to fit into our current API.

[Ralith]

I think this is fundamentally a dup of #2024. See discussion there. We definitely want this feature, but it's not trivial.