Add FTDI bit-bang GPIO output support

[ozan956]

This adds FTDIGPIO resource, an FTDIGPIODriver, and a small USB agent. The feature is intended for boards where FTDI data-bus pins are used as simple digital outputs, for example to control straps, enables, reset-like signals, or other board control lines during tests.

This targets FTDI asynchronous bit-bang GPIO on the data bus pins. It does not target CBUS GPIO, EEPROM-configured ACBUS functions, or generic MPSSE GPIO.

Supported devices are:

• FT2232-style devices, 0403:6010, interfaces 1..2
• FT4232H, 0403:6011, interfaces 1..4
• FT232H, 0403:6014, interface 1

Each interface exposes an independent 8-bit data bus. index selects bit 0..7 on that interface.

For writes, the agent first reads the current pin byte, modifies only the requested bit, and writes the byte back. This avoids unintentionally changing other bits on the same FTDI interface. get() reads the current level of the configured output line.

I also would like to support input pins with direction feature of those chips in future, so #1458 would be very helpful.

I tested the driver with a FT232H device (0403:6014).

Checklist

• Documentation for the feature
• Tests for the feature
• The arguments and description in doc/configuration.rst have been updated
• Add a section on how to use the feature to doc/usage.rst
• Add a section on how to use the feature to doc/development.rst
• PR has been tested
• Man pages have been regenerated

[codecov]

Codecov Report

❌ Patch coverage is 99.49239% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 61.7%. Comparing base (64723a5) to head (5ba2f44).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
labgrid/driver/ftdigpiodriver.py	98.4%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1931     +/-   ##
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+ Coverage    61.0%   61.7%   +0.6%     
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  Files         182     184      +2     
  Lines       14881   15077    +196     
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+ Hits         9083    9308    +225     
+ Misses       5798    5769     -29     

Flag	Coverage Δ
3.10	61.6% <99.4%> (+0.6%)	⬆️
3.11	61.7% <99.4%> (+0.6%)	⬆️
3.12	61.6% <99.4%> (+0.6%)	⬆️
3.13	61.6% <99.4%> (+0.6%)	⬆️
3.14	61.6% <99.4%> (+0.6%)	⬆️

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