Introduce add-on metadata

[kaikreuzer]

This is a follow-up on eclipse-archived/smarthome#4503 suggesting to define a OH-INF/addon.xml file that could include different metadata about the add-on.

In the case of bindings, this should be a replacement for the binding.xml file.

Content of the OH-INF/addon.xml file should be:

• name - mandatory - The add-on name, no reference to specific solution (e.g. openHAB). Unclear if type should be included ("Air Quality" vs. "Air Quality Binding")
• description - mandatory - A description of the purpose, specialization and limitations of the add-on, should be between 160-240 characters.
• type - mandatory - One out of a counted list, e.g. "Binding", "Automation", ...
• keywords - optional but recommended - Keywords (categories) out of an open list to filter against, e.g. "heating, lighting, graphing"
• connection - optional - highlight if a binding/automation/... interacts locally or if it connects to an online service (seldom or continuously)
• countries - optional - a comma-separated list of ISO 3166 2-letter country codes for which this add-on is applicable

The metadata must be made available through the REST API, so that it can be consumed by the UI.

[ghys]

The metadata must be made available through the REST API, so that it can be consumed by the UI.

I have made some proposals in #2070.

[lolodomo]

@J-N-K @ghys : are the metadata connection and countries already supported by MainUI ?