Please add Software UART to ESP32

**Describe the problem you have/What new integration you would like**
Hey, i'm trying to use on my ESP32 all three hardware UARTs + a fourth Software UART. This Software UART won't work.
As seen on the UART Sourcecode, there is no Software UART on ESP32.
On the [docs](https://esphome.io/components/uart.html#hardware-uarts) it reads like there is Software UART on all ESPs.

**Please describe your use case for this integration and alternatives you've tried:**
I try to query two Victron Solar Charger, an Daly BMS and an Victron Smart Shunt via UART. In the Future maybe additional a Nexion Display via UART. The ESP32 is capable of this, i tried it already on Arduino Code.

**Additional context**
ESPHome should fall back by itself if all HW UARTs a occupied to SW UART.

Start with the UART source code and the hardware-UART documentation linked in the request. Determine how ESP32 UART allocation currently works and define what support for a fourth software UART and automatic fallback should mean. Done means the documented behavior matches the implementation and the requested ESP32 use case is supported.

Written by the indexing model from the issue text.