JIRA: https://jira.spring.io/browse/INT-3991
* Add `IntegrationGraphController` `@RestController` over `IntegrationGraphServer` bean
* Add `@EnableIntegrationGraphController` and `<int-http:graph-controller>` to register
`IntegrationGraphController` if the `DispatcherServlet` is in classpath.
* Also register the `IntegrationGraphServer` bean from the same place, if that isn't presented in the application context yet.
* Allow to configure the "root" `path` for the `IntegrationGraphController` as a placeholder value via
`spring.integration.graph.controller.request.mapping.path` property
* Add tests for both `@EnableIntegrationGraphController` and `<int-http:graph-controller>` cases based on the `MockMvc`
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