Divide sink classes to separate modules

Closes #264 (closed)

@edgarrmondragon - Do you mind reviewing this? There should be basically no code changes but this splits the module so that SQL-type sinks can be added more cleanly.

Note: the top-level sinks.py is now deleted, but the MR positions this as a rename:

singer_sdk/sinks.py → singer_sdk/sinks/core.py

Also, the sinks/__init__.py still exposes the class names with the intent that this change be non-breaking.