Draft: bluez5: Hardwired SCO support

Current implementation of SCO source/sink for HSP/HFP profiles send and receive audio stream through the HCI link.

But some Bluetooth chipsets have direct audio channel connection and are able to hardware encode/decode the SCO stream.
In this is the case the Bluetooth chipset can be connected directly to an audio DSP or to the audio channel of a modem (e.g. for the PinePhone).

This commit adds a monitor property to select hardwired SCO and propagate it to the devices.
When the HSP/HFP profile is selected this just acquire the transport, i.e. create the SCO connection, instead of creating the SCO source and sink nodes.

Transport release is already done when switching to another profile.

It is responsibility of the session manager to correctly set up the audio path between host, BT chipset and DAC or modem.