Add index-level privilege for force merge

[elasticmachine]

Original comment by @ppf2:

Force merge can be dangerous. Sometimes even admins have to be blocked from running it, and only allow super/smarter admins to run it. With the move to high level privileges, force merge action is now part of a broader manage privilege at the index level. Perhaps we can create force_merge privilege and remove it from the generic manage privilege so that they have to grant it explicitly for specific users?

[albertzaharovits]

Resolved in #50643