[5.x] JSON Serialization

[jasonvarga]

This is aiming to solve issues where certain objects aren't serialized to JSON correctly. Instead, you'd see just {}.

Relevant:

• [4.x] Make entries json serializable #9514
• Handle query builder when using json_encode or to_json. #6229
• The to_json modifier doesn't handle nested query builders #5866

Closes #5866