feat: added update checker

[tarna]

Proposed changes

Adds an update checker that fetches the latest version from hangar and if the versions don't match, it sends a message to players who have the permission attollo.update when they join. Resolves issue #61.

I wasn't sure what type of message you wanted so I just put some random one. You guys can change it to what you guys prefer.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING.md
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

N/A

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