Do not assume that the sid sent to client is the account sid

[edgrosvenor]

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Twilio allows for the creation of api keys and tokens that can (arguably should) be used in lieu of the account sid and its token. Using the account sid and its token puts credentials in the wild that have the power to create new api keys and perform account-level operations that are not required or often appropriate for a system that just sends messages. This PR simply adds another configuration key sid that defaults to account_sid if it's not set. So now the user can configure the package to use an api key sid and its token, but if not, the package will continue to function exactly as it did before.