Add instance cache for custom key store provider registration

[johnnypham]

Currently, custom key store providers can only be registered into a global cache at the application level. These changes add a public method to register providers into an instance cache at the connection-level, allowing users to maintain their own set of providers in the same application. This cache takes precedence over the global cache and can be set more than once.

public class SqlConnection
{    
       // Current API: registers into global cache
       public static void RegisterColumnEncryptionKeyStoreProviders(IDictionary<string, SqlColumnEncryptionKeyStoreProvider> customProviders);

       // Added API: registers into instance cache
       public void RegisterColumnEncryptionKeyStoreProvidersOnConnection(IDictionary<string, SqlColumnEncryptionKeyStoreProvider> customProviders);
}