Add automated_security_fixes for github_repository

[voanhduy1512]

Resolves #588

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After the change?

• Add ability to turn on dependabot through automated_security_fixes field

Pull request checklist

• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• [ ] Yes
• [x] No

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[kfcampbell]

This behavior is currently managed by the github_repository_dependabot_security_updates resource. What's the thought process behind putting it inside the repository resource? The repo resource is already pretty bloated, and inserting new functionality here means the existing resource will conflict, so I'm pretty hesitant to add this.

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