chore: URLSessionHTTPClient logging & code cleanup

[jbelkins]

Description of changes

Some cleanup of URLSessionHTTPClient code and logging.

• All HTTPClient logging of routine (i.e. non-error) events reduced from .info to .debug log level
• The delegate callback for urlSession(_:dataTask:didReceive:completionHandler:) is changed from the Swift concurrency form to the pre-concurrency form, to be consistent with all other delegate callbacks
• Logging added for description of request body
• Various code cleanup

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

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