Add in-memory provider that uses IMemoryCache

[wazzamatazz]

What problem does the feature solve?

Consider adding a caching provider that uses Microsoft.Extensions.Caching.Memory.IMemoryCache as its store. This could be used as an alternative to the built-in MemoryCacheLayer.

The default MemoryCache implementation of Microsoft.Extensions.Caching.Memory.IMemoryCache includes features such as automatic compaction if the cache exceeds a given size limit. It would be useful to be able to piggyback onto these capabilities while still using CacheTower's flexible layering.