Add endpoints2.0 middleware and param binding generation.

[isaiahvita]

All endpoints2.0 middleware and parameter binding generation lives here, while hooks were added for builtin generation on consumer SDKs

[aajtodd]

suggestion: It might be better to collapse these into one method that returns a type responsible for these different things.

e.g.

default void endpointBuiltinHandler(): EndpointBuiltInHandler


interface EndpointBuiltInHandler {
    default void renderEndpointBuiltInField(GoWriter writer) { }

    default void renderEndpointBuiltInInvocation(GoWriter writer) {}

    default void renderEndpointBuiltInInitialization(GoWriter writer, Parameters parameters) { }
}

Grouping these things together reduces the API surface of GoIntegration, it also helps in finding all the various methods that may need overridden for related functionality.