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

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• 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

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of
them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before
merging your code.

• 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

[tarna]

added all the new requested changes

[tarna]

have turned the update messages to constants like requested

[TheMeinerLP]

Have you tested locally ?

[tarna]

Have you tested locally ?

Yes, I have. When testing locally, I temporarily change the plugin version to something lower.

[TheMeinerLP]

LGTM