Replace "tcp-only" with a more generic option

Bind has a tcp-only option to force interaction with a server using TCP only.

server <address> {
    ...
    tcp-only yes;
    ...
};

This option was enough when DNS was using UDP and TCP only (Do53), however as it becomes possible to carry DNS traffic with more transports, we might need to replace this option with a more generic one which could specify the desired transport explicitly, e.g.:

server <address> {
    ...
    transport tcp; # or "tls", or "quic" (in the future), etc
    ...
};

When the option is omitted, it means use the default (Do53 - the current behaviour).

This issue, in a way, mirrors #2776 (closed).