Support passing `ClientConfiguration` to web identity credentials provider.

[teo-tsirpanis]

Issue #, if available:

Description of changes: This PR updates the constructor of STSAssumeRoleWebIdentityCredentialsProvider to accept an optional ClientConfiguration parameter.

Also the provider no longer tries to determine itself the region to use and instead always defers to the region specified in the config. This shouldn't have any practical effect since the default constructor of ClientConfiguration has a similar (and slightly more extensive) logic either way.

Check all that applies:

• [X] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.)
• [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.
• [X] Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• [ ] Linux
• [X] Windows
• [ ] Android
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• [ ] IOS
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