This allows builds know about all schemes - but only have the protocol
implementations for those actually built-in.
It further allows multiple protocols to reuse the same protocol setup
and functions for both TLS and non-TLS implementations instead of
needing two (or more) structs.
The scheme information is now in 'struct Curl_scheme' and all the
function pointers for each scheme/protocol implementation are in struct
Curl_protocol.
The URL API now always work with all known protocols.
Closes#20351
8edc0338f3