WIP: JSON support

[xiongjiwei]

The JSON type is supported in tidb but not GA yet. This issue will trace the JSON GA project.

Goal

make the basic function is available

Non-Goal

multi-valued index Partial update

Task

• [ ] json-path-syntax compatibility and enhancement
• [ ] cast to/from string
• [ ] cast to/from number
• [ ] cast to/from date
• [ ] cast to/ from NULL
• [ ] comparison between json

function(expression) index and generate columns

• [ ] create function index using json_unquote

all the issues