MINOR: ssl: load sctl from buf OR from a file

MINOR: ssl: load sctl from buf OR from a file

The ssl_sock_load_sctl_from_file() function was modified to
fill directly a struct cert_key_and_chain.

The function prototype was normalized in order to be used with the CLI
payload parser.

This function either read  text from a buffer or read a file on the
filesystem.

It fills the ocsp_response buffer of the struct cert_key_and_chain.