Determine robust cache key mechanism for BlobContainerClientFactory and CosmosContainerFactory

Currently, `BlobContainerClientFactory`'s `GetCacheKeyForContext` override just serializes the `BlobContainerConfiguration` to JSON as an easy way to generate a key that is guaranteed to be different if the configuration is different. This is pretty inefficient though. And it can have startup costs too, because it will load in everything required for JSON.NET serialization. (In some environments that's priced into startup costs, but not everywhere.)

We should consider doing something that has the same effect a lower cost.