Firebase wrappers such as `onRequest`, `onCreate`, etc return callable
handlers with some important additional data in properties which is
being used by `firebase-cli` while deploying the functions.

So when we wrap such functions naively, these properties are not
visible anymore. So I decided to circumvent it with a
`new Proxy(fn, { apply: ... })` trick.
This proxy object will redirect all property accesses to the original
function but calling behavior will be re-defined.

Also added some workarounds for firebase emulator (`firebase
emulators:start`)

- Need to wrap `fn.__emulator_func` too. See code comments.
- Need to create a new domain because unlike `functions-framework`,
  firebase emulator doesn't create a domain.

Fixes getsentry#3023.

Also this commit removes a dependency on `@google-cloud/functions-framework` package
since it's used only for importing types but these types are also needed for TypeScript
users and we cannot force them to install an entired functions-framework
as a dependency.
Instead, I moved all the types from it right in this package and also add a
*runtime* dependency `@types/express`. Fixes getsentry#2997.

It's not enough to include it only in `devDependencies` because without
it TypeScript users are getting type errors.