Add Type Annotations to `asgiref` Module Functions

[davitacols]

I suggest adding type annotations to the following functions in the asgiref module:

1. is_double_callable
2. double_to_single_callable
3. guarantee_single_callable

Proposed Changes:

• Add type annotations to function parameters and return types in the specified functions.

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 into key functions.

[andrewgodwin]

Sounds reasonable - I'll happily take a pull request for this.

[davitacols]

I am happy you found this useful. Best.