Replace abstract handlers, customer handlers, type annotations, refactoring.

[jordaneremieff]

A big refactoring and change to how the handlers will be designed going forward. I'm starting with the something that will make it easy to support custom handlers that could (potentially) be extended to support multiple vendors beyond AWS. The custom handlers can probably be used to support some other AWS use-cases more generally as well, so I've marked them as "provisional" until I'm more clear on how this will end up.

Changes:

• Drop support for Python 3.6.
• Abstract handler pattern replaced with lambda handler Protocol implementation to cover handling of handler inference, request body, scope parsing, and response handling.
• New provisional configuration to allow passing in handlers that comply with the protocol to take priority over the default handlers. Allow user provided handlers #177
• Dataclasses removed, didn't really see the need for the dependency at this point.
• New type annotations for events and various typing fixups.
• Update README and requirements.
• Added a more descriptive runtime error message for when a handler cannot be inferred since there have been a number of issues and discussions related to this in the past (Unable to determine handler from trigger event #237, errorMessage: Unable to determine handler from trigger event #217, errorMessage: Unable to determine handler from trigger event #218, KeyError: 'requestContext' #126), so hopefully this can help a bit there.