drivers: gpio_rpi_pico: Add gpio_get_config API

Implement the `gpio_get_config`

N.b. adding this API results in a new test failure in
`test_gpio_config_trigger`. This suggests that there is some kind of
dependency between this and the now-enabled `pin_get_config` test cases.
Note that this adds a read-only API, it is unlikely to be the cause of
the failure.

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