Pass service name to Smithy endpoint providers

[Renz456]

Issue #, if available:

Fixes #3907.

Description of changes:

Pass the service name stored by AwsSmithyClientT to InitBuiltInParameters. This allows Smithy-based clients such as STS to resolve service-specific and global endpoint URL configuration consistently with non-Smithy generated clients.

Add a core unit test that verifies Smithy client construction selects the service-aware endpoint-provider overload and supplies the expected service name.

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.
• 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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[sbiscigl]

yep looks goods, for process reasons i need to pull your commit onto a branch I own and can merge it from there. the commit will still credit you.

[sbiscigl]

merged #3909 with your commit on it, thanks for the contribution