feat: trigger event upon paying bill

[Z3rio]

An event was added to the server & client side to show whether a bill was paid or not.

Why is this neat to have?
Personally I want to add this as it allows other scripts to listen to when esx_billing bills are paid.
Instead of having to periodically fetch the billing database table and check the data there.

Forexample: I have a parking enforcement job script, which sends bills via esx_billing, where I'd want to sync the data for whether the bill is paid or not in my script.

[Gellipapa]

Please do this request changes, but code is cool. Thanks for this feature.

[Gellipapa]

Thanks for fixes.