Add account ID to static credentials providers

[alexgromero]

This PR introduces account ID to SharedCredentialProvider and ConfigProvider. The update allows users to specify an aws_account_id variable either in the shared credentials file, config file, or during client creation. The account ID will then be associated with the credentials when they are loaded.

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##             account-id-endpoint-routing    #3348   +/-   ##
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[jonathan343]

I think these changes look good. The only thing I'd suggest is adding a test that verifies the correct precedence for account_id if specified in multiple places such as the shared config file, environment variable, client creation parameter, ...