[I2C] Add support for multiple I2C interfaces

[tonhuisman]

Resolves: #4592

Features:

• Add support for multiple I2C interfaces (ESP32 only!), default 2, max. 3.
  • Uses software-multiplexing, to avoid having to change (and test!) many libraries and plugins.
  • Explicitly disabled for ESP8266.
• Also enables to have an optional I2C multiplexer per I2C interface.
• Per directly addressed I2C hardware (External Time Source, WatchDog hardware, PCF/MCP addressing without task like monitor command) an I2C Interface selector is available.

TODO:

• Reset default I2C settings on upgrade
• Testing
• Documentation updates

[uzi18]

@tonhuisman @TD-er In fact for esp8266 i2c is software based so it could be also enabled via compatible soft library implementation.

[tonhuisman]

@tonhuisman @TD-er In fact for esp8266 i2c is software based so it could be also enabled via compatible soft library implementation.

Yes, I know, but we have decided some time ago that for ESP8266 we are feature-complete, and want/need to motivate users to switch to ESP32 based boards.
This feature is mostly implemented to support the newer generation ESP32 models (S3, C5, C6, P4) with multiple I2C interfaces, on boards that actually have I2C hardware available at different GPIO pin pairs, like the LilyGO T-Beam S3. (That board even has dual SPI busses implemented, so another challenge to address 😅)