Use AwsCredentialsProvider and AwsRegionProvider interfaces for bean …

[shduke]

…declaration instead of their Chain classes

Declaring these beans using their interfaces will make them easier to override.

AwsCredentialsProviderChain is a final class, which prevent AOP proxying (e.g. it prevent using the @MockBean annotation directly).

Using the @Replaces() annotation works; however, these beans should be thought of (and are logically being used) as a black box implementation of their Provider interface.

AwsRegionProviderChain is not a final class; however, if AwsCredentialsProviderChain is changed then AwsRegionProviderChain should be changed as well for consistency.

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