S3 Crt ClientConfiguration: expose Aws::Client::ClientConfiguration

[grrtrr]

Since the S3CrtClient is a sub-class of the AwsClient, expose the configuration needed by the parent class.

Many API calls use the parent class, so preserving user configurations is important.

This now uses slicing instead of a conversion operator, so that the parent struct only sees the value relevant to it.

Since it initializes the base struct first, this also addresses #1768.

Issue #, if available: Fixes #1882, #1768 and #1654.

Description of changes:

Check all that applies:

• [X] Did a review by yourself.
• [X] Added proper tests to cover this PR. (If tests are not applicable, explain.) Tested with large application in our repo
• [X] Checked if this PR is a breaking (APIs have been changed) change.
• [X] 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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