iBeacon Transmitter - Ability to change transmission frequency

**Is your feature request related to a problem? Please describe.**

By default, the BLE iBeacon transmitter transmits once a second. Depending on transmission power, this can be quite draining for the battery.

**Describe the solution you'd like**

I'd love to be able to change the transmission frequency of the BLE iBeacon transmitter to a custom value, for instance 2 seconds.
I understand that longer than 1 second is often not going to be optimal for room presence, but if the default remains at 1 second then increasing the value is a deliberate choice made by the user.
You could even add a warning for values higher than 1 second, if you're worried about people not understanding and raising issues for bad configuration.

**Describe alternatives you've considered, if any**

Reducing transmission power, but 1) this isn't always an option, depending on setup, and 2) if transmission power is already `ultraLow` but the user wants further battery savings, this will aid in that.

**Additional context**