Feature/kcl 1193 custom elements in sdk

[dzmitryk-kontent-ai]

Motivation

KCL-1193

Checklist

• [x] Code follows coding conventions held in this repo
• [x] Automated tests have been added
• [x] Tests are passing
• [x] Docs have been updated (if applicable)
• [x] Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

To test it manually you should generate classes from project which contains Custom Element type. Custom Element should be generated as string property