Checklist

• I've searched the issue tracker for similar requests

Is your feature request related to a problem? Please describe.
We occasionally encounter TLS clients that send invalid SNIs. As some other TLS libraries accept such invalid SNIs, our customers expect us to work with their non-compliant TLS clients as well.

Describe the solution you'd like
I think a simple solution would be to add the option to configure the rustls server to accept (and ignore) invalid SNIs in ClientHello (similar to what was done for IP-addresses-as-SNI in #1881).

Describe alternatives you've considered
Modifying ClientHello before passing it to rustls was suggested, but given that the TLS handshake transcript will go into calculating the session master key, I don't think it's possible.