Deprecate IndexConfig in favor of type-specific configs

[clee704]

Up to v0.4.0, users can create a single type of index using IndexConfig.

We are adding more index types. When there are multiple index types, IndexConfig can be confusing to users because it is actually just for a single type of index. Therefore, it would be better to change the class name of IndexConfig to CoveringIndexConfig to match the name of the index it creates, CoveringIndex.

Since it will break existing users' programs, to avoid unnecessary inconvenience to users, we can keep IndexConfig in v0.x and change the name from v1.0. Meanwhile, we can add deprecation warnings to IndexConfig - this is optional.