No way to access global variables from .cpp file in "includes"

### The problem

with the includes directive it's possible to link .cpp files into the project. However, to be able to usefully implement code in this cpp file, it should be possible to access the global variables that are generated by ESPHome.

Example:
```
esphome:
name: "${device_name}"
includes:
- test.h
- test.cpp
on_boot:
priority: 200
then:
- lambda: !lambda test();
```
In test.cpp:
```
void test()
{
ESP_LOGD("test", "%f", id(input_fan_speed).state);
}
```

If ESPHome would a header file with externals to link the generated globals, this code would run.

### Which version of ESPHome has the issue?

git dev branch dd. January 5th, 2023

### What type of installation are you using?

pip

### Which version of Home Assistant has the issue?

N/A

### What platform are you using?

ESP32

### Board

nodemcu

### Component causing the issue

core

### Example YAML snippet

_No response_

### Anything in the logs that might be useful for us?

_No response_

### Additional information

_No response_