ADD Policy list

Add policy list retrieval.
I haven't tested it, but I think that there are more attributes (`PolicySetId`) and children nodes (`Target`) in a PolicySet, and maybe some of them are mandatory.

Take a look at <https://github.com/wso2/commons/blob/master/balana/modules/balana-core/src/test/resources/conformance/3/policies/IIIA014Policy.xacml3.xml>.

Could you please check the conformance against the XSD?