examples: add GPIO drive strength config parser and htool gpio sweep …

[qwerty-10434]

…command

Add a zero-dependency configuration file parser and an interactive 'htool gpio sweep' subcommand in examples/htool_gpio.{c,h} to step through GPIO drive strengths (0..3) across SPI interfaces (spidev, spihost0, spihost1) for signal integrity (SI) validation.

• Parse per-interface 6-signal pad mappings (.clk, .cs, .d0, .d1, .d2, .d3) using DIO0..DIO15 and MIO0..MIO46 pad names (matching 'htool gpio set_drive_strength --dio/--mio') and drive strength lists (.drive_strengths).
• Require all 6 SPI signals to be explicitly mapped for each configured interface, and reject unrecognised keys, invalid values, or lines exceeding the maximum line length.
• Read back each pad immediately after setting its drive strength and warn on stderr if the readback value does not match.
• Save the initial drive strengths of all 6 pads before sweeping and restore them on every exit path (normal completion, 'q', EOF, a failed set, or SIGINT/SIGTERM/SIGHUP).
• Support '--dry_run' ('-n') to step through combinations without connecting to a RoT device or sending host commands.
• Add unit tests in examples/htool_gpio_test.cc and a reference configuration file in examples/gpio_sweep.conf.example.