X509Utils provides tools for working with X.509 certificates and keystores, as used in the BIP 70 payment protocol.
For more details on this, see PaymentSession, the article "Working with
the payment protocol" on the bitcoinj website, or the Bitcoin developer guide.
Returns either a string that "sums up" the certificate for humans, in a similar manner to what you might see
in a web browser, or null if one cannot be extracted. This will typically be the common name (CN) field, but
can also be the org (O) field, org+location+country if withLocation is set, or the email
address for S/MIME certificates.