Add Async API integration

[dsanchor]

Add Async API integration in a step-by-step doc and within an API.

You could use httpbin service to mock the async behavior:

<inbound>
    <set-backend-service base-url="https://httpbin.org" />
    <rewrite-uri template="/redirect-to" copy-unmatched-params="false" />
    <set-query-parameter name="url" exists-action="append">
        <value>https://httpbin.org/status/200,302</value>
    </set-query-parameter>
    <base />
</inbound>
<backend>
    <base />
</backend>
<outbound>
    <base />
    <retry condition="@(((IResponse)context.Variables["var"]).StatusCode == 202)" count="10" interval="5">
        <send-request mode="new" response-variable-name="var" ignore-error="false">
            <set-url>@(context.Response.Headers["location"][0])</set-url>
            <set-method>GET</set-method>
        </send-request>
    </retry>
    <return-response response-variable-name="var" />
</outbound>
<on-error>
    <base />
</on-error>