Add custom policy external integration

[dsanchor]

Add custom policy external integration to both step-by-step and a provisioned API.

As an example, httpbin service could be used as backend mock:

    <send-request mode="new" response-variable-name="validationresponse" timeout="20" ignore-error="true">
        <set-url>https://httpbin.org/status/200,403</set-url>
        <set-method>GET</set-method>
    </send-request>
    <choose>
        <!-- Check active property in response -->
        <when condition="@((int)((IResponse)context.Variables["validationresponse"]).StatusCode == 403)">
            <!-- Return 403  -->
            <return-response>
                <set-status code="403" reason="Custom validation failed" />
            </return-response>
        </when>
    </choose>