Fix AttributeAddModal to make JSON values less concerning

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

• "Object" values should be actually called "JSON" values
• "JSON" value should not be a default, I'm not sure why we're actually switching to JSON every time someone wants to provide a value
• Attribute preview should be rendered always by reusing existing components, not only for Rich attributes. In addition, right now JSON values are not parsed at all and they're passed to preview as strings

Related with #907

Before change it looks like that:

What is the new behaviour?

After change it will look like that: