Add SDKOption for default Credentials Provider Chain

[MitchellKagerKodiak]

Issue #, if available: related to 150; this is kind of a workaround whereby developer can set their default chain to include STSProfileCredentialsProvider. Just opened an issue / feature request 2814.

Description of changes: If provided in SDKOption, initialization will resolve closure and set that value in static variable. If not provided, configure static variable with DefaultAWSCredentialsProviderChain. Most usages of DefaultAWSCredentialsProviderChain throughout clients has been replaced with GetDefaultCredentialsProviderChain.

I could use some help writing test cases, and I probably missed a couple places where I need to replace DefaultAWSCredentialsProviderChain.

I know this looks tedious to review; but, all of the logic changes are in src/aws-cpp-sdk-core. Everything outside of that is just changing existing usages of DefaultAWSCredentialsProviderChain => GetDefaultCredentialsProviderChain across the services' clients.

Check all that applies:

• [x] 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.
• [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.

• [x] Linux
• [ ] Windows
• [ ] Android
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• [ ] IOS
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