Custom typing generator

[T4rk1n]

Add custom typing for component props, previously hardcoded for dcc.Graph.
Component libraries developer can add a file dash_prop_typing containing one or two variables:

• custom_imports: dict[ComponentName, list[str]] import statement to be copied at the top of the component class definition.
• custom_props: dict[ComponentName, dict[PropName, function]] for custom props. The function signature is: def generate_type(type_info, component_name, prop_name) -> str.

The module path can be changed with the dash-generate-components argument -t or --custom-typing-module.