Content directory smart contracts: Languages, categories and licenses

[Lezek123]

If we'd only rely on the initial idea described in #1816, then in order to update the set of avilable languages / categories / licenses we would need to update the metadata json schema that includes them (ie. in form of json-schema enum type), create a PR, publish new version of content directory lib and CLI and possibly update and re-deploy all other projects that rely on the previous schema (ie. query node and possibly Atlas once it includes uploading / channel management)

The easiest way to avoid that would be have some representation of categories and languages in smart contracts instead. This seems to be the only way to handle "choice type" video/channel properties for which we want curators / lead to be able to add new possible choices without the need to change any actual code (so perhaps that would also include licenses?)

In that case each language, category and license would have it's own distinct metadata (provided as json string) and the contracts would expose methods to add / update / remove entities, executable by the lead.