Use the LoadSecurityTokenAssertion
method to parse the XML elements of policy assertions that reference a specific
token type. The ISecurityTokenAssertion returned from this
method can then be passed to the LoadTokenFromSecurityTokenAssertion
create an instance of the specific security token that
satisfies the conditions of the security token policy assertion.
This method should return null if this function is not supported in a
particular security token manager implementation.
Override this method when the security
token is used with policy.