multi: add multi->proto_hash, a key-value store for protocol data

multi: add multi->proto_hash, a key-value store for protocol data

- add `Curl_hash_add2()` that passes a destructor function for
  the element added. Call element destructor instead of hash
  destructor if present.
- multi: add `proto_hash` for protocol related information,
  remove `struct multi_ssl_backend_data`.
- openssl: use multi->proto_hash to keep x509 shared store
- schannel: use multi->proto_hash to keep x509 shared store
- vtls: remove Curl_free_multi_ssl_backend_data() and its
  equivalents in the TLS backends

Closes #13345