Create schema for PBDLSharedMasterNode

[ivan-015]

The schema for PBDLSharedMasterNode should contain all the attributes of PBDLNode, in addition to the following:

• availableOverrideProperties - a list of overrideProperty objects of the Shared Master.

The overrideProperty object shall contain the attributes of PBDLNode in addition to the following:

• properties - a list of the properties of the element that can be overridden.

Below is a shortened example of what a PBDLSharedMasterNode should look like:

{
	"type": "pbdl_shared_master_node",
	"UUID": "<UUID>", // What PBDL `instances` will reference as "symbolID"
	"availableOverrideProperties": [
		{
			"type": "overrideProperty",
			"UUID": "<UUID>", // What PBDL `overrideValues` will reference as "propertyUUID"
			"properties": [
				{ // sample property
					"type": "color",
					"value": "0x00000000" // default value. For sketch, we need to do a lookup inside symbol master's layers to find this value
				},
				{
					"type": "corner_radii",
					"value": [35,35,35,35] // default value. For sketch, we need to do a lookup inside symbol master's layers to find this value
				}
			],
		}
	],
}