Add support for PCF8575 I/O expander

**Motivation:**
The [PCF8575](https://www.nxp.com/docs/en/data-sheet/PCF8575.pdf) is a widely used 16-bit I²C I/O expander, commonly found in embedded systems.
Adding support for this device would extend compatibility for I/O expansion over I²C and benefit users needing more digital I/Os.

**Scope:**
- [ ] Implement an expander for the PCF8575
- [ ] Support input and output modes
- [ ] Expose a consistent API with existing expanders
- [ ] Consider exposing low-level utilities (e.g., bit-shifting helpers)

**Impact:**
New feature: no impact on existing functionality.