SFTPSensor - add file_pattern to template fields

[JstFlip]

Added file_pattern to template fields because I would like to use airflow variables such as {{ ds }} inside the file_pattern when e.g. looking for file with the execution date in its name.

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[boring-cyborg]

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[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.