Create copies of mutable properties on X509Certificate2

Create copies of mutable properties on X509Certificate2

Export(Cert) returned the original byte array from the PAL. If a
caller mutated the result of the export, they would be mutating the
underlying representation of RawData in the PAL. To be consistent
with Windows and Linux, we return a copy in the PAL.

Fix OpenSSL PAL to prevent side effects between mutable
SubjectName.RawData and Subject properties (similarly for Issuer)