expose connect `app` from http transport

[shanejonas]

I need to app.set('trust proxy') as seen here since heroku is behind a proxy.

https://github.com/open-rpc/server-js/blob/fc7ace81241a45610e73be7f2b39556b32d8f608/src/transports/http.ts#L20

[piboistudios]

This would also be nice in general, as I'd like to use OpenRPC alongside OpenAPI. Since connect/express apps are just handler functions, having access to this would let users plug OpenRPC into their existing projects.