[Proposal] SpiController and ISpiController

[shaggygi]

Similar to the GpioController, there needs to be a mechanism to determine the best SpiDevice to use.

Updated 6/24/2019

Proposed Changes

1. Add New SpiController

The creation of devices with a new static helper method that attempts to pick the best SPI device for OS app is executing on. This static method will be added to a new sealed partial class respective to OS.

// SpiController.Linux.cs
public static SpiDevice Create(SpiConnectionSettings settings) => new UnixSpiDevice(settings);
// SpiController.Windows.cs
public static SpiDevice Create(SpiConnectionSettings settings) => new Windows10SpiDevice(settings);

[shaggygi]

Referencing #126

[joperezr]

We can probably bundle this up with #118 since they will be super similar, so I'm fine with closing this one in favor of having only one issue for both Spi and I2c controllers.

[shaggygi]

@joperezr Please reopen this and assign to me. New proposal notes are in the works. Hope to post soon. Thx.