[I2C] Add support for multiple I2C buses

[tonhuisman]

Resolves: #4592

Features:

• Add support for multiple I2C buses (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 bus.
• Per directly addressed I2C hardware (External Time Source, WatchDog hardware, PCF/MCP addressing without task like monitor command) an I2C Bus selector is available.

Extra fixes/features:

• Some selectors appended undesired (default) for - None - values (fix).
• Show GPIO Pin status for Wifi Status LED and Reset Pin, as configurable on the Hardware page (feature).
• Show overlapping PSRAM and other (conflicting) uses in GPIO pin-selectors and on Hardware page (Boot states overview), ESP32-only, n/a on ESP8266. (fix/feature)

TODO:

• Reset default I2C settings on upgrade
• Testing
  • Additional testing: monitor,mcp,<n> on the PCF & MCP Direct I/O bus as configured on the Hardware page.
• 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 😅)