Additional certificate utility methods #67

Add the following useful methods to CertUtil:

1. encodeCert - produce the PEM-encoding given an X509Certificate object
2. subjectDN - produce a string version of the subject of the given X509Certificate object
3. generateCert - Produce a self-signed X509Certificate object given a key pair and validity period as a Duration

For (2), consider flags or overloaded variants that optionally produce output containing spaces; for example:

System.out.println(selfSignedCert.getSubjectX500Principal().toString());
CN=middleware-test, OU=Middleware, O=Virginia Polytechnic Institute and State University, L=Blacksburg, ST=Virginia, DC=vt, DC=edu, C=US
System.out.println(selfSignedCert.getSubjectX500Principal().getName());
CN=middleware-test,OU=Middleware,O=Virginia Polytechnic Institute and State University,L=Blacksburg,ST=Virginia,DC=vt,DC=edu,C=US

Comparing DNs by string representation is fraught with peril, but providing both outputs could be helpful for simple cases.

For(3) the method should support both RSA and EC keys.