[B+C] Add setKnockbackStrength for arrows. BUKKIT-5103

The Issue:
None: just add more stuff in the api.

Justification for this PR:
Less nms in plugins.

PR Breakdown:
Use the vanilla fonction setKnockbackStrength on arrow entity, must be positive (mc check that) and have no limits.

Relevant PR:
https://github.com/Bukkit/CraftBukkit/pull/1287

JIRA Ticket:
https://bukkit.atlassian.net/browse/BUKKIT-5103