Add Type Annotations to `asgiref` Module

[davitacols]

Description: I propose adding type annotations to the functions in the asgiref module to enhance code readability and provide better tooling support.

Changes Proposed

• Add type annotations to function parameters and return types in the asgiref module.

Benefits:

• Improved code readability.
• Better tooling support for developers using the asgiref module.

Additional Context: This change aims to enhance the maintainability and clarity of the asgiref codebase by incorporating type annotations.

[andrewgodwin]

We already have type annotations for most of the main functions, so I'm going to close this as not-a-bug; if there's specific functions that aren't typed that you think would help, feel free to open a specific issue about those.