Endpoints 2.0 Integration pre-work (#2063)

* Split endpoint resolution middleware into two parts & refactor endpoint generation

* Endpoints 2.0 Integration pre-work

This PR does a 3 bits of pre-work ahead of ep2 integration:
1. Split endpoint resolution into two separate middlewares:
  1. A smithy native middleware that applies URI and headers
  2. An AWS middleware that applies the auth schemes
2. Add vendorParams support to the ProtocolTestGenerator so that protocol tests can insert a region.
3. Simplify endpoint resolution logic by allowing `make_operation` to fail when an endpoint cannot be resolved.

* Back out previous change to insert endpoint directly into the bag

* backout changes to property bag

* Update changelog & add more docs

* Fix AWS test

* Fix test