add metadata field for publishing

[lamebrain]

Add the ability to pass arbitrary metadata when publishing an MPAK and pass through to the Update Service. This metadata can be used by the device to determine if it should download and apply the update.

• Update CLI with metadata param add metadata param for package publish Meadow.CLI#331
• Update UI with metadata param
• Update publish endpoint to add the metadata to the MQTT message
• Update UpdateService to pass in metadata on the OnUpdateAvailable event add metadata field for Update Meadow.Contracts#115