Improve the performance of the FusionCacheProvider

[0xced]

Storing the caches in a dictionary upfront allows to retrieve the cache in O(1) instead of O(n).

Using Lazy<IFusionCache> allows to preserve the semantic of throwing when accessing the (incorrectly configured) caches.

Also, the DependencyInjectionTests were very slightly improved:

• Using GetRequiredService instead of GetService plus the null-forgiving operator because an InvalidOperationException is always better than a NullReferenceException, even in unit tests.
• A new assertion was added to ensure that the default IFusionCache instance is always the same no matter how it's retrieved, either through IFusionCacheProvider or directly by requesting the IFusionCache.

[jodydonetti]

Hi @0xced I updated this branch based on the new main, and pushed some changes which build upon your work and can make it even better.

Basically instead of storing an IEnumerable to iterate on at every GetCache() call, I just stored either a single LazyNamedCache in case there was only one, or null in case they were more than 1: in this way no need to an iteration per-call, less cpu usage and less allocations.

Finally I've made the tests a little better around these scenarios.

Thoughts?

[0xced]

Looks good to me too!