Custom networking components

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

ESPHome has hardcoded dependency to AsyncTCP.
It would be great if at least api server can use other communication options.
E.g. Having ESPHomeTCP interface - fallback connectivity scenarios can be possible. Or new ways of communication using serial, i2c, lo-ra...

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

For my home automation I would rather try to use wired-serial communication with devices as much as possible. Would someone else be interested in alternative networking implementations?

Following things might become possible:
- usb attached devices
- one wire mesh
- long range radio
- vpn, proxyng
- mocked api clients for unit testing

**Additional context**

I am ready to start working on this kind of component
1. extract AsyncTCP-like interface for esphome just enough for api server
1. Add default implementation using AsyncTCP
1. Add possibility to create custom c++ networking components.