Add internal features for device capabilities

[usbalbin]

Today we have lots of places with code like this:

#[cfg(any(
    feature = "stm32g471",
    feature = "stm32g473",
    feature = "stm32g474",
    feature = "stm32g483",
    feature = "stm32g484",
    feature = "stm32g491",
    feature = "stm32g4a1",
))]
use self::config::ExternalTrigger345;

What do you think about adding a set of internal device features so something like the above instead could be written like

#[cfg(feature = "has_adc345")]
use self::config::ExternalTrigger345;

or similar?

Here is a list of device capabilities.

BTW is there any way to make these features internal/private since they are supposed to be selected as a direct consequence of selecting mcu feature?