feat(bluetooth): Proper bond management, identity support for non-splits

* Add `bt` behavior that can be used to perform certain actions,
  such as next/prev identity, reset identity, etc.
  NOTE: Multiple identities is only supported for non-split shields,
  due to missing Zephyr identity functionality for dual
  central/peripheral devices.
* Proper bond reset tied to action, that honors peripheral bonds,
  so folks can reset and pair to other hosts, without breaking
  bonds between splt halves.

app/dts/behaviors/bluetooth.dtsi

@@ -0,0 +1,9 @@
+/ {
+	behaviors {
+		bt: behavior_bluetooth {
+			compatible = "zmk,behavior-bluetooth";
+			label = "BLUETOOTH";
+			#binding-cells = <2>;
+		};
+	};
+};