HTTP function emulator forcibly handles all OPTIONS requests, and seems to assume they are all CORS preflight requests

### [REQUIRED] Environment info

**firebase-tools:** 13.0.2

**Platform:** macOS

### [REQUIRED] Test case

Any OPTIONS request to an HTTP function.

### [REQUIRED] Steps to reproduce

1. Author any trivial HTTP function that logs anything upon invocation
2. Run the emulator with it
3. Send it an OPTIONS request, for example:

```
curl \
-H "Origin: https://example.com" \
-H "Access-Control-Request-Method: GET" \
-H "Access-Control-Request-Headers: X-Requested-With" \
-X OPTIONS \
--verbose \
[function url]
```

### [REQUIRED] Expected behavior

I expect to see a log message for the invocation of the OPTIONS request.

### [REQUIRED] Actual behavior

The emulator internally handles the OPTIONS request and doesn't invoke the function code. On top of that, it seems to assume that the OPTIONS request is a CORS preflight request. It always sends back a response that looks like this:

```
< HTTP/1.1 204 No Content
< x-powered-by: Express
< access-control-allow-origin: https://example.com
< vary: Origin, Access-Control-Request-Headers
< access-control-allow-methods: GET,HEAD,PUT,PATCH,POST,DELETE
< access-control-allow-headers: X-Requested-With
< content-length: 0
< date: Sat, 23 Dec 2023 21:20:36 GMT
< connection: close
```

Note also (as a second, separate problem here) that the access-control-allow-origin header is **always** exactly what was provided in the request Origin header, regardless of any actual cors configuration that would send otherwise. Also the access-control-allow-methods is always the same and never what the function wants to send (because it can't actually handle the cors response).

This behavior is problematic because I can't test my own OPTIONS handling, or any 3rd party CORS handling libraries. I have to deploy the code to see a proper OPTIONS invocation with the response that I want.

I understand this behavior makes it easy to get started working with HTTP and callable functions, but it's not accurate emulation, and leads to potential disappointment or frustration upon deployment.