feat(policy): adds support for match subject request to get entitlements without FQN scopes

[jakedoublev]

Closes #348

Notes about performance concerns at scale are captured under a separate new issue: #365

Example request (using provisioned fixtures):

grpcurl -plaintext -d '{"subject_properties":[{"external_field":"team_name","external_value":"ShinyThing"}]}' localhost:9000 policy.subjectmapping.SubjectMappingService.MatchSubjectMappings

Example response:

{
  "subjectMappings": [
    {
      "id": "1748761a-bd8c-4b23-8560-16ba7a181f19",
      "attributeValue": {
        "id": "c2140825-0969-44c9-8dd6-5d7e0a856b9c",
        "value": "blue",
        "active": true
      },
      "subjectConditionSet": {
        "id": "10d03422-7eae-43b9-ac3b-d10400171858",
        "subjectSets": [
          {
            "conditionGroups": [
              {
                "conditions": [
                  {
                    "subjectExternalField": "team_name",
                    "operator": "SUBJECT_MAPPING_OPERATOR_ENUM_IN",
                    "subjectExternalValues": [
                      "CoolTool",
                      "RadService",
                      "ShinyThing"
                    ]
                  },
                  {
                    "subjectExternalField": "org_name",
                    "operator": "SUBJECT_MAPPING_OPERATOR_ENUM_IN",
                    "subjectExternalValues": [
                      "marketing"
                    ]
                  }
                ],
                "booleanOperator": "CONDITION_BOOLEAN_TYPE_ENUM_AND"
              }
            ]
          }
        ]
      },
      "actions": [
        {
          "standard": "STANDARD_ACTION_DECRYPT"
        }
      ],
      "metadata": {
        
      }
    }
  ]
}