Display extension pricing information

Two user stories, from different perspectives

As a user I want to know whether the extension I am considering to install is free to use or requires a subscription. The extension mode shall display the value of the pricing attribute. The subscription shall be conducted directly with the publisher, and Qt shall have no role in that arrangement. 

As a publisher I want to define whether my extension is free to use or requires a subscription

Possible technical realization

Add Pricing, an string-typed field into the metadata-schema, with two supported values: “free” and “free trial” - these values mimic those used by Visual Studio Code. 

Validation: is present, is not empty. 

OPEN: this could be in the base-schema as well, but keeping it in the metadata allows the publisher to change the pricing schema with a version update.