feat: async filter interface

[jg-rp]

This pull requests adds support for async filters via an optional filter_async method on class-based filters.

When rendered in an async context, the filter_async method of the following filter will be awaited, instead of calling __call__.

class SomeFilter:
    """A mock class-based filter implementing the async filter interface."""

    def __call__(self, val: object) -> str:
        return "Hello, " + str(val)

    async def filter_async(self, val: object) -> str:
        return "Goodbye, " + str(val)