Add GPIO initial internal pull-down resistor state

[apcountryman]

Add GPIO initial internal pull-down resistor state (::picolibrary::GPIO::Initial_Pull_Down_State).

• The Initial_Pull_Down_State enum class should be defined in the include/picolibrary/gpio.h/source/picolibrary/gpio.cc header/source file pair
• The Initial_Pull_Down_State enum class should have an underlying type of std::uint_fast8_t
• The Initial_Pull_Down_State enum class should have the following enumerators:
  • DISABLED,: Disabled
  • ENABLED,: Enabled
• A stream insertion operator should be defined for the Initial_Pull_Down_State enum class in the include/picolibrary/testing/automated/gpio.h/source/picolibrary/testing/automated/gpio.cc header/source file pair
• Documentation