Add DurationsAsCount preprocessor

[cacharle]

This allows to serialize std::chrono durations with their count alone instead of the default count and unit.

This follows the same pattern as UnderlyingEnums by creating an empty preprocessor and using it as a flag to change the duration parsers behavior.

The main reason to have this is to be more compatible with external applications that usually format durations as plain integers and have the unit in the field name (e.g duration_ms: 1000).

[liuzicheng1987]

@cacharle thanks for the contribution! I am merging this one as well.