Create CertificateConverters class to aid in X509Certificate property conversions

Much like the [RsaKeyConverters](https://github.com/spring-projects/spring-security/blob/master/core/src/main/java/org/springframework/security/converter/RsaKeyConverters.java) there is a need to be able to convert an `X509Certificate` from a string in a property

From [PR Review #7260](https://github.com/spring-projects/spring-security/pull/7260#discussion_r316791196)