MINOR: ssl: OCSP functions can load from file or buffer

MINOR: ssl: OCSP functions can load from file or buffer

The ssl_sock_load_ocsp_response_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 a base64 from a buffer or read a binary file
on the filesystem.

It fills the ocsp_response buffer of the struct cert_key_and_chain.