Support Custom Servlet for OpenAPI Doc generation

[ayushr2]

Currently, there is a requirement that the servlet-class has to be either com.google.api.server.spi.SystemServiceServlet or com.google.api.server.spi.EndpointsServlet for the openAPI doc generation tool to generate docs for the endpoints servlet.

Code Reference:

endpoints-java/endpoints-framework-tools/src/main/java/com/google/api/server/spi/tools/WebXml.java

Lines 53 to 57 in 39b7374

	String findService = "/web-app/servlet" + "["
	+ "servlet-class = 'com.google.api.server.spi.SystemServiceServlet'"
	+ " or "
	+ "servlet-class = '" + EndpointsServlet.class.getName() + "'"
	+ "]/init-param[param-name = 'services']/param-value/text()";

However in some usage cases we want to extend EndpointsServlet with a custom Endpoints Servlet which can handle some middleware-esque work. In that case our servlet-class will be neither of the above but will have inherited one of them. For example,
https://github.com/TranDuongTu/tduhack-dsa/blob/master/src/main/webapp/WEB-INF/web.xml#L22-L29

This is not my project but the author is doing something similar to what I am doing.