Add GPIO internally pulled-down input pin concept

[apcountryman]

Add GPIO internally pulled-down input pin concept (::picolibrary::GPIO::Internally_Pulled_Down_Input_Pin_Concept) and associated mock (::picolibrary::Testing::Automated::GPIO::Mock_Internally_Pulled_Down_Input_Pin).

• [x] The Internally_Pulled_Down_Input_Pin_Concept class should be defined in the include/picolibrary/gpio.h/source/picolibrary/gpio.cc header/source file pair
• [x] The Mock_Internally_Pulled_Down_Input_Pin class should be defined in the include/picolibrary/testing/automated/gpio.h/source/picolibrary/testing/automated/gpio.cc header/source file pair
• [x] Internally pulled-down input pins should be expected to support the following operations:
  • [x] Internally_Pulled_Down_Input_Pin_Concept() noexcept;
  • [x] Internally_Pulled_Down_Input_Pin_Concept( Internally_Pulled_Down_Input_Pin && source ) noexcept;
  • [x] ~Internally_Pulled_Down_Input_Pin_Concept() noexcept;
  • [x] auto operator=( Internally_Pulled_Down_Input_Pin && source ) noexcept -> Internally_Pulled_Down_Input_Pin_Concept &;
  • [x] void initialize( Initial_Pull_Down_State initial_pull_down_state = Initial_Pull_Down_State::DISABLED ) noexcept;: Initialize the pin's hardware
  • [x] auto pull_down_is_disabled() const noexcept -> bool;: Check if the pin's internal pull-down resistor is disabled
  • [x] auto pull_down_is_enabled() const noexcept -> bool;: Check if the pin's internal pull-down resistor is enabled
  • [x] void disable_pull_down() noexcept;: Disable the pin's internal pull-down resistor
  • [x] void enable_pull_down() noexcept;: Disable the pin's internal pull-down resistor
  • [x] auto is_low() const noexcept -> bool;: Check if the pin is in the low state
  • [x] auto is_high() const noexcept -> bool;: Check if the pin is in the high state
• [x] The Mock_Internally_Pulled_Down_Input_Pin class should include a movable mock handle
• [x] Documentation