Add policyExport feature

features/policyExport.feature

+As user of an external system
+I want to have an exported result of the policy in JSON-LD format
+So I am able to verify the result of policy evaluation
+
+Scenario: Export of the policy evaluation result
+Given policy configured for being evaluated for export
+When I evaluate the policy
+Then I get a successful response
+And the response is in a JSON-LD format
+And the response contains proof section signed by TSA
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