Enable Swift 5.10 strict concurrency checking, correct issues

[jbelkins]

Describe the bug

Swift 5.10 introduces "strict concurrency checking", intended to identify the use of unsafe types and practices in Swift concurrent code; see:
https://www.swift.org/documentation/concurrency/

Enable this mode for development, and either:

• Correct the issues as they are identified, or
• File tickets to correct the issues that are identified.

Expected Behavior

N/A

Current Behavior

N/A

Reproduction Steps

N/A

Possible Solution

No response

Additional Information/Context

No response

AWS SWIFT SDK version used

N/A

Compiler and Version used

N/A

Operating System and version

N/A

[jbelkins]

This must be done to support Swift 6, which we plan to do at GA, because all of these "strict concurrency checking" warnings become compile errors in Swift 6.