tests: gpio_api_1pin: rpi_pico2: Support using external pulldowns

Extend gpio_api_1pin so that tests can require a test fixture to provide
an external pulldown resistor to the board under test. Use the new
test-gpio-external-pulldown device tree binding to define where that
GPIO is, and, finally, add a device tree overlay for the Raspberry Pi
Pico 2 board that defines where the pulldown provided by the fixture
will be.

Tested locally using `--fixture gpio_external_pull_down` when running
Twister on the command line, or by creating and using a Hardware Map
file, in combination with a modified Pico 2.

Signed-off-by: Andrew Featherstone <andrew.featherstone@gmail.com>