Add an API to get the signer of an OCSP response

Add a new function OCSP_resp_get0_signer() that looks in the
certs bundled with the response as well as in additional certificates
provided as a function argument, returning the certificate that signed
the given response (if present).

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4573)