Add device features support

[jjcarstens]

Start of device features (previously "extensions") which allow for specialized features on device to report data and interactions safely outside the update mechanism.

Requires nerves-hub/nerves_hub_link#228

Based on conversations the last while, these are some of the core design decisions so far:

• NervesHub selectively tells a device what features to enable
• Features are off by default on device
• Features should have no affect on socket connectivity or ability to update a device

With that in mind, this is the basic communication cycle for supporting features on device.

sequenceDiagram
    participant Device as Device (NHL)
    participant NH as NervesHub

    Device ->> NH: Connect
    NH ->> NH: Check `features_allowed?`
    NH ->> Device: Send "features:get"
    Device ->> NH: Join "features" channel with [%{<feature> => <version>}]
    NH ->> NH: Determine features to enable based on report
    NH ->> Device: Send "features:enable" to with list of features to enable

    loop For each feature to enable
        Device ->> NH: Report "<feature_name>:<event>" where event is enabled or error
    end

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TODO: All the bits to selectively decide what features can be enabled for a device

[lawik]

Looks like a good approach to me. Want me to flesh out the allow mechanisms as a branch off of this branch or something?

Or we merge this once we tidy up the dbg and TODO notes?

[lawik]

I think the only devices that use Geo are ours from testing and we could absolutely break that contract and fix those devices.

The functionality has been added in nerves_hub_web but the other end only exists in branches of nerves_hub_link extension stuff.