[DRAFT] Pure python request pipeline

[JordonPhillips]

This is a draft of the non-codegen request pipeline. It's at a point where it looks like what it'll look like in the final draft, but there's some other components that need to be updated still, namely:

• Auth needs to be decoupled from HTTP and params need to be standardized
• Endpoint resolution needs to be decoupled from HTTP and params need to be standardized.
• Error classification for retries needs to be handled in a robust way. smithy-java is just handing that off to the retry strategy, but that's not good enough IMO. I think there needs to be a shared responsibility:
  • smithy-core needs to perform default classification based on the model.
  • Transports and/or protocols need to classify their own errors, perhaps through a classify method, based on the response.
  • retry strategy can then reclassify if it likes
• Protocols need to wrap event streams. smithy-java handles this through serde (e.g. write_event_stream / read_event_stream) but Python doesn't have some standard thing like Flow to fall back on so it's not quite so simple.
• Related to the above, event interfaces need to be moved into smithy-core
• Requests need to be reset during retries, including seeking seekable bodies. IMO the Request should handle that with a retryable and reset method.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[nateprewitt]

I think this looks fine conceptually.