Feature: auto generate virtual columns for Variant data type

[b41sh]

Summary

Virtual columns can greatly improve performance by extracting internal json fields and storing them separately, but now virtual columns need to be manually created by users, which is inconvenient to use. We need to support automatic generation. By extracting the schema of each json fields in the block, we can determine whether it is the same type and automatically generate virtual columns.

• Add the address of the virtual column file at BlockMeta to determine whether the file is generated during refresh.
• Derive the schema of the json fields to determine whether each field has the same type, and if so, it is suitable to generate virtual columns.
• Set the generation rule to automatically refresh to generate virtual columns.