Bluetooth: host: Add application control of PHY change procedure

Add application control of initiating phy change procedure and it's
parameters.
The reasons for allowing the application control over the PHY:
 - Allow changing to Coded PHY.
 - Application may change PHY to react to changes in environment
   to balance throughput and range.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>