support raspberry as building target

**Describe the problem you have/What new integration you would like**

Currently, ESPHome supports ESP32 and ESP8266 as building targets, and thanks to YAML syntax file, It's easy to make sensors and switch work by only a few lines. I am thinking if it is possible to support raspberry pi as a building target. YAML syntax file would make it easy to control raspberry pi's GPIO, SPI, I2C bus, and MQTT or native API would make it easy to integrate raspberry to home assistant.

**Please describe your use case for this integration and alternatives you've tried:**

I am doing some DIYs connecting raspberry to some sensors, cameras, buttons, screens. I have to deal with GPIOs, i2c, and MQTT. It is borning and makes me miss ESPHOME's YAML syntax.

**Additional context**

Start by surveying how ESPHome currently builds for ESP32 and ESP8266, then compare the Raspberry Pi requirements named here: GPIO, SPI, I2C, sensors, cameras, buttons, screens, MQTT, and the native API. No files or tests are identified in the request; done would require defining and implementing a complete Raspberry Pi build target and validating the listed integrations.

Written by the indexing model from the issue text.