Isolate DatatypeConverter Usage

[jzheaux]

In JDK 11, javax.xml.bind is dropped and so Spring LDAP needs to find an alternative to DatatypeConverter in order to be runnable in JDK 11+.

Because Spring LDAP 2.3.x compiles to JDK 6, it's not as simple as changing to use java.util.Base64. Instead, the code should:

• check if the java.util.Base64 class is present. If so, use it
• otherwise, continue using DatatypeConverter

Since this is passive, it can be introduced in a point release.