[ROUTING] Add rebalance enabled allocation decider

This commit adds the ability to enable / disable relocations
on an entire cluster or on individual indices for either:

 * `primaries` - only primaries can rebalance
 * `replica` - only replicas can rebalance
 * `all` - everything can rebalance (default)
 * `none` - all rebalances are disabled

similar to the allocation enable / disable functionality.

Relates to #7288
Extremely minor grammar thing, these should be:

```
all rebalancing is allowed
no rebalancing is allowed
primary rebalancing is allowed
replica rebalancing is disallowed
replica rebalancing is allowed
primary rebalancing is disallowed
```

I'd also recommend `forbidden` instead of `disallowed` because it's much less likely to mix up with `allowed` at a quick glance.