Permit Join timeout is hardcoded to 254 sec (4:14)

[gusarov]

Today the default "permit join" button duration is hardcoded to 254 seconds (4:14) which neither represent human friendly number nor serves reasonable purpose, since I found myself quite often in need for a bit more time while fiddling with new gadget.

I also verified that if I send zigbee2mqtt/bridge/request/permit_join {"value": true, "time": 300} it works, so there is no hard limit for this number

Let's have a configuration option for this number somewhere. Even yaml config is fine without UI just to have this possibility.