DBUS: Implement the "channel" arguments in "EnergyScan" method

[caipiblack]

Hello,

It's possible to start an EnergyScan to a specific channel using ot-ctl commands like this:

# Duration: 200ms
# Channel: 20
ot-ctl scan energy 200 20

• We would like to do the same with DBUS, but currently only the duration parameter is present
• The two methods call the same function: otLinkEnergyScan() but the DBUS variant, always read the channel from otPlatRadioGetPreferredChannelMask()

The scan method of ot-ctl is calling otLinkEnergyScan() here

The EnergyScan method of DBUS API is calling otLinkEnergyScan() here

We need it for the PCBA tests, we would like to read the RSSI on a specific channel (instead of all, to speed up)

The difficulties here are maybe just to know how to make the parameter channel optionable in the DBUS API.

There is maybe a temporary method, but I don't like it:

• Calling ot-ctl channel preferred xxx before and after the scan triggered by DBUS..