Improve template capabilities of EMR job and step operators

[oripwk]

The EmrCreateJobFlowOperator and EmrAddStepsOperator operators already support receiving job_flow_overrides and steps arguments as templates. However, one of the benefits of templating is the ability to refer to a file, which is not currently supported. A good example of an existing operator which leverages file templates is BashOperator, which applies this principle to its bash_command parameter.

Adding such support will improve the usability of the operators greatly, since at the moment the only way to do it is manually invoking Jinja to read the files before passing the arguments to the operators.

This PR simply adds the proper file extensions to template_ext field, and converts the resulting string into a Python dictionary.

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request!