Expose Express App in server.setupFn

[Martinsos]

In server.setupFn, users could benefit quite a bit from having access to the Express App -> that way they can do some customization, like custom routes or custom middleware!

To pass them app into the setupFn, we merely have to modify line await {= serverSetupJsFnIdentifier =}() in data/Generator/templates/server/src/server.ts to be await {= serverSetupJsFnIdentifier =}(app).
That should be the core of it!

Additional stuff to do:

1. Modify docs to mention this.
2. I am not sure if we need to do anything to ensure it is correctly typed by TS, we should possibly look into that. Also, can this be a breaking change then, if it modifies type of serverFn?

The whole issue is quite simple!

[Martinsos]

Done by @shayneczyzewski !