Support split keyboards

[daystram]

This PR adds support for split keyboards. For transport, only a basic implementation of UART is implemented.

Configuration for Quadax Rift is also added.

Changes summary:

• Add split scanner (based on BasicVerticalSwitchMatrix)
• Add remote server executor and client invoker
  • Simple flow control implemented
  • No retries or circuit breaker, thus not reliable with higher baud rates
  • Memory allocator increased to 16 KB (from 2 KB)
• Add UART remote transport
• Add split side and mode detection
  • Side detection uses a dedicated pull-down pin
  • Mode detection is based on USB host detection, not VBUS as femto does not have a VBUS detection pin
• Breakdown initialization to two steps
  • Required to detect host for split mode detection
  • Moved some peripherals away from RTIC's local resources to parameter passed to spawned tasks
• Refactor keyboard configuration module
• Add debugging module
• Add run script for both probe and U2F runners for RP2040
• Add keyboard configuration for Quadax Rift

Known issues:

• RGB matrix and rotary encoder does not have remote invocation implemented yet
• High client latency due to high packet serialization overhead
  • Scan result serialization implementation is naive and not performant
  • Achieving 1 kHz polling rate (1 ms main loop period) is difficult with the current configuration
    • At 115200 Baud rate, input scan latency is about 7 ms
    • At 230400 Baud rate, input scan latency is about 4.2 ms, but transmission is sometimes not reliable