Add character’s experience and level-up threshold to /characters

[scy]

I love tracking a character’s progress over time, and I’m already able to retrieve the wallet, inventory and level. However, in higher levels it takes a large amount of time to reach the next one. Therefore I think it would be useful to be able to track the experience bar of a character.

I’m proposing three new values that are available via /characters if the token has progression permission:

{ experience: {
  current: 123456,
  this_level: 1168459,
  next_level: 1236659
}}

These allow you to retrieve the current absolute amount of experience as well as the amount required to reach the current and the next level.

You can use these to calculate an XP bar percentage: (current - next_level) / (next_level - this_level) * 100.

Once a character reaches level 80, XP gained will instead count towards masteries (if the player owns HoT/PoF), which are tracked account-wide and not per character. Therefore they are out of scope for this PR. For level 80 characters, current and this_level will become and stay the same number (maximum XP, currently 4,860,359), while next_level will change to false, meaning that there is no next level.

An alternative way to implement this would be to just return a single number for experience, without providing this_level or next_level. Applications would then need to consult the XP table or a (not yet implemented) separate endpoint providing it. This would be easier to implement on the server side, but harder for clients.