Implement never_cache decorator

[florimondmanca]

Currently, when CacheMiddleware is applied then all application endpoints will be cached according to the cache.ttl. Users may want to not cache a specific endpoint, e.g. because its response should always be fresh.

The proposed API is a new @never_cache decorator:

from datetime import datetime
from asgi_caches.decorators import never_cache

@app.route("/datetime")
@never_cache
class DateTime(HTTPEndpoint):
    async def get(self, request):
        return JSONResponse({"time": datetime.now().utcformat()})

Alternatives that were considered:

• Use @cached(ttl=0): this would work, as currently if the TTL is zero we do not cache at all (see Don't cache if TTL is 0 #10), but it is not the most intuitive API.

To implement this feature:

• Add the @never_cache decorator in decorators.py. You may want to implement it as a proxy to @cached(ttl=0), if it turns out to work.
• Add a test that if the decorator is applied, the endpoint is not cached even if the application is wrapped in CacheMiddleware.

(Note: the situation where both @cached and @never_cache isn't within the scope of this issue. It will be dealt with as part of #16.)