Wire async I/O path into AWSClient and SmithyClient for the CRT HTTP client

[sbiscigl]

Description of changes:

Adds an opt-in asynchronous I/O path for the CRT-backed HTTP client and wires it through both client bases.

This change adds the asynchronous I/O request path for the CRT HTTP client, serving as the entry point that a future code generation change will build on to make service operations non-blocking. The path is controlled by a new CMake option that is off by default, so nothing changes for existing customers until it is explicitly enabled. Adding this plumbing now lets the follow-up code generation work wire services onto it without further changes to the client core.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• Checked if this PR is a breaking (APIs have been changed) change. Not breaking: new opt-in API and CMake flag; default-off path is unchanged.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

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