feat: add function for billing per identifier instead of only source

[Z3rio]

This PR aims to primarily add a function to actually bill players based on their identifier, instead of only being able to bill by their source/player id.

Along making these changes, I also felt like incorporating server exports for this would be great, instead of juts having events that can only be triggered from the clientside. Hence the minor logic refactoring in the normal bill function.

[Gellipapa]

Hi! Please do this little refactor but code cool thanks for improvment to esx_billing.

[Gellipapa]

Hi! Thanks for fix, but please fix little problems.

[Z3rio]

@Gellipapa Why should if not xTarget then return end be preferred?
Just checking if xTarget isn't nil (aka the way its done right now) is a lot more logical, more readable, does the same thing, and takes less lines.

[Gellipapa]

@Gellipapa Why should if not xTarget then return end be preferred? Just checking if xTarget isn't nil (aka the way its done right now) is a lot more logical, more readable, does the same thing, and takes less lines.

@Z3rio Hi! The basic logic is to deny because it will always be much more readable to get the actual result than to always write it in IF, look it up. In most cases it's most readable if you exclude everything line by line and at the end you only have the actual result before you always exit the code. This is a common and used method in software development.

https://dev.to/jpswade/return-early-12o5

[Gellipapa]

Thanks for fixes i merged.