Add support for IntegrationEndpoints

[micovery]

https://cloud.google.com/apigee/docs/api-platform/reference/api-proxy-configuration-reference#integrationendpoint

<ProxyEndpoint name="default">
    <Description/>
    <FaultRules/>
    <PreFlow name="PreFlow">
        <Request>
            <Step>
                <Name>my-set-integration-request-policy</Name>
            </Step>
        </Request>
    </PreFlow>
    <Flows/>
    <PostFlow name="PostFlow"/>
    <HTTPProxyConnection>
        <BasePath>/integration-endpoint-test</BasePath>
        <Properties/>
    </HTTPProxyConnection>
    <RouteRule name="default">
        <IntegrationEndpoint>my-int-endpoint</IntegrationEndpoint>
    </RouteRule>
</ProxyEndpoint>

Actual integration Endpoint file:

<IntegrationEndpoint name="my-int-endpoint">
    <AsyncExecution>true</AsyncExecution>
</IntegrationEndpoint>