Execution Interceptors

[kneekey23]

We need to design how to offer the ability for a customer to add middlewares to the stack to do things like append to the User Agent or add a header to the request. Go SDK has examples of this.

Acceptance criteria

• Build a hello world app that adds a unique piece of middleware to each step, and verify proper execution of each of the custom middlewares owned by the hello world app.

[ganeshnj]

This is part of reference arch and once the design is approved. We can implement this.

[smivij]

g3DoAzTyXlo6/Smithy-Interceptors

[sichanyoo]

Related issue: #1130
The temporary fix for Amplify team should be scrapped as part of SRA interceptor implementation.

[dayaffe]

PR #593 in smithy-swift should be reverted when SRA Interceptors is implemented.