servlet: configurable methodNameResolver

[panchenko]

instead of #11825

[ejona86]

There should be at least documentation mentioning that using this method is basically guaranteed to be incompatible with reflection, and that using https://grpc.github.io/grpc-java/javadoc/io/grpc/HandlerRegistry.html when possible is preferred.

[kannanjgithub]

I didn't follow. The HandlerRegistry has methods for looking up ServerMethodDefinition but the method name to use for look up would have the context prefix removed using the method name resolver. As the HandlerRegistry and the method resolver are for two different purposes, when do we face the incompatibility with reflection?

[long76]

@panchenko u forgot protected on constructor GrpcServlet(ServletAdapter servletAdapter)

original solution #11825 (comment)

[panchenko]

@long76 you don't need changes to the constructor, it's more flexible if registering servlets from ServletContainerInitializer - #11825 (comment)