feat(split): Add peripheral is bonded/connected to central

[ReFil]

For easy use by displays and indicators it would be helpful to be able to see if any peripheral at an index is connected and bonded, follow on from #2032

[caksoylar]

Nice, it might be useful for a single display (on central) setup on splits.

[caksoylar]

Is there an existing event to hook up to in order to listen to peripheral changes? If not, might be a good opportunity to add one here.

[ReFil]

There is but it doesn't work quite right. This gets addressed in #2036 as it's needed for that functionality

[petejohanson]

Thoughts on a single API that returns an enum with the state? The "is connected" makes no sense if we don't even have an address bonded to that index, for example, so a single API to just query the "peripheral state" makes more sense, IMHO.

[ReFil]

I think that's a good idea in general, my only concern would be that's a divergence from how it's currently handled for central->host connections and peripheral->central connections which both use a pair of bool return functions. Would it be better to change all of them over to a consistent API with one BLE_CONN_STATE enum with connected, bonded but not connected and unbounded values?

[ReFil]

Thinking about this more i'm concerned changing the other functions would leak to breakage of module and config repo custom display implementations. A stopgap would be implement enum return, then change the bool return functions to reference that enum return function for some period of time then remove them, similar to how the encoder changes were handled