Add wrapper for BossBar packet

[Elikill58]

This PR add a wrapper for BOSS_BAR packet.

I pre-answer few questions:

• I made my own Color enum to prevent switch/case to read color. It also help to other developer to only use right colors
• The constructor isn't with all parameters as they are different between each actions
• For the flags, I didn't convert them to something else that byte as it seems to changed multiple times. Also, I'm reading as UnsignedByte but there isn't writeUnsignedByte, I tried writeShort but that was too much.
• In general, I used description available here and tested on my own server

I suggest you to Squash and merge. That's would be perfect.

[booky10]

Using adventure's bossbar classes would be a lot nicer to work with

[Elikill58]

Using adventure's bossbar classes would be a lot nicer to work with

That's can be difficult as only one action has informations about the full bossbar, else it's only a part of it. Should I add something that is a partial bossbar, return null when not full or do nothing?

[booky10]

I meant using Bossbar.Color, etc. Not constructing adventure bossbar objects

[Elikill58]

Oh yes, it's good idea. I just made changes about this :)

[booky10]

could you also parse the flags sent in the packet?

[Elikill58]

I'll check. It will require more testing as it's more versions depending

[Elikill58]

That's it! :) (Note: please don't forget to squash, it's better)

[retrooper]

Thanks for your PR.