Implement cache invalidation

[jmccand]

Is your feature request related to a problem? Please describe.
fastapi-cache2 doesn't have builtin support for cache invalidation, but it would be useful to have it for Shubble.

Describe the solution you'd like
We should be able to implement cache invalidation by defining custom invalidation methods like await FastAPICache.get_backend().delete(key).

[RichardJSun]

This should already be supported using FastAPICache.get_backend().clear(namespace) without any customized methods I believe. To use it with the cache decoration we could specify the namespace.

[RichardJSun]

https://github.com/long2ice/fastapi-cache/blob/b2143ef85e2868dace363e432a0683675577bf81/fastapi_cache/backends/redis.py#L24

[jmccand]

Implementing in #269