Extracting function event validator to its own class.

[byawitz]

What does this PR do?

Closes#6218

Test Plan

1. Added FunctionEventValidatorTest.php which is the same as EventValidatorTest.php except that any event that starts with functions. will return false.
2. Tested it on a temporary local Appwrite instance, and I was able to add the functions.* events to a webhook but not to a function.

Related PRs and Issues

#6218

Checklist

• Have you read the Contributing Guidelines on issues?
• If the PR includes a change to an API's metadata (desc, label, params, etc.), does it also include updated API specs and example docs?

[stnguyen90]

Would you please also uncomment the functions test in the event validator unit test?

[byawitz]

Done