feat: add PHY type

[acouvreur]

Platform will now be able to implement Device.GetPHY() by returning this PHY type extracted from the platform specific types.

I will follow up with a implementation for Windows to be able to read the current device Phy.

Few notes:

• This could definitely be in gap.go
• I could add an unknown phy enum, but the default is PHY1M for all BLE devices

[deadprogram]

This could definitely be in gap.go

That seems like the most logical place to add it.

I could add an unknown phy enum, but the default is PHY1M for all BLE devices

Valid point.

[acouvreur]

@deadprogram I have moved the PHY definitions to gap.go

[deadprogram]

Thanks for the addition @acouvreur now squash/merging.