This adds a converter for the SHA-2 family, supporting SHA-224, SHA-256
SHA-384 and SHA-512.
The converter relies on the OpenSSL implementation, thus only being available
when HAProxy is compiled with USE_OPENSSL.
See GitHub issue #123. The hypothetical `ssl_?_sha256` fetch can then be
simulated using `ssl_?_der,sha2(256)`:
http-response set-header Server-Cert-FP %[ssl_f_der,sha2(256),hex]
