Add a `content_rating` field to inscriptions

[lgalabru]

In the explorer, we want to be able to filter out certain inscriptions based on their content, and this could be a nice feature to expose to other developers. The content moderation will most likely be manual operated by our team - to be defined cc @chresko.

[chresko]

@lgalabru We should decide if we'll support this or close it out. Thoughts?