Add support for iterating over regions and accounts in CLI

[ablackrw]

Issue #, if available: Prototype implementation for #7173

Description of changes: The goal of this change is to provide a simple way to execute a particular command in multiple regions and accounts. The implemented mechanism is roughtly an order of magnitude faster in execution time than iterating using a shell loop.

For account iteration, the organizations:ListAccounts facility is leveraged to provide a list of accounts to iterate over. The sts:AssumeRole service is then used to obtain access to the identified accounts using role switching.

For region switching, a client is created for each enumerated region. This facility uses a new command line switch rather than overloading the --region switch to allow the account iteration to function correctly.

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