Fix warnings in AST Loader for Resource and Operation Shapes with mixins

[hpmellema]

Description of changes:
Updates the AST loader to allow both Operation and Resource shapes to have a mixin property without throwing a warning.

Previously the following warning was being emitted on building the source projection:

      |
50 |         "example.weather.operations#OperationWithMixin": {
      |                                                          ^
     = Expected an object with possible properties of `errors`, `input`, `output`, `traits`, `type`, but found additional properties: `mixins`
SUCCESS: Validated 299 shapes (WARNING: 1)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[syall]

Looking at the spec for mixins, services and other shapes might also have this warning, will need to make sure each of the shape types have the addMixins() call.