Implement public assets

[richturner]

There is a requirement to be able to mark an asset as public and then it should be possible to query/get these public assets without being authenticated.

Having a 'public' user with hardcoded credentials that are then baked into the frontend doesn't seem great plus would require direct access grant.

Simple solution maybe:

• have a 'public' flag on asset
• remove Roles allowed from AssetResource::queryAssets and to programmatically limit the query to assets with public=true if user is not authenticated (realm must be specified in the request otherwise fail i.e. don't allow public assets in the master realm)

[richturner]

Done a temporary implementation that uses the AssetType to identify public assets, anything that starts with 'urn:openremote:publc' can be accessed using the /asset/public/query API endpoint.

A realm must be specified in the request path and only protected attributes and meta can be retrieved.