Modify `PrivateKeyAccessRule` to make serialization possible

[zentron]

Another small step towards extracting full framework functionality of Calamari.

This change modifies the PrivateKeyAccessRule class to remove the IdentityReference property as this class will need to be serialized when sent between the separate processes.

Although the class already supports serializing the identity as a string, this assumes that it is in an NTAccount format and does not as easily support the SecurityIdentifier type which can be passed into this class constructor internally.