Implement definition of the form additional metadata in a separate YAML file

Problem/Motivation

Adding simple form-related things like description to the data schema looks okay.

However, storing some heavy form-styling metadata (like weight, CSS classes, HTML code for the suffix and the prefix, description about how to fill the field, etc) in the data schema does not look good!

Proposed resolution

Seems it'll be good to store the form rendering metadata in a separate YAML file, with a reference to the original data schema. It will be kind of a template, describing how to extend the form for the typed data.

Remaining tasks

In this issue, let's try to invent something good for this.