Add response filtering options

[JWCook]

• Add a filter() method to get only a subset of cached responses, similar to requests-cache's filter()
• To start, filtering out expired responses would be the most useful option
• Other options (like older_than) could be added if/when needed
• Support the same option(s) for get_urls()

Example usage:

async for response in session.cache.filter(expired=False):
    print(response)

urls = [url async for url in session.cache.get_urls(expired=False)]