Add JSON access query support

[rickpr]

When attempting to query JSON using this syntax, I receive the following error:

sqlalchemy.exc.UnsupportedCompilationError: Compiler <snowflake.sqlalchemy.base.SnowflakeCompiler object at 0x13f6cefd0> can't render element of type <function json_getitem_op at 0x1103c4790> (Background on this error at: http://sqlalche.me/e/13/l7de)

This means my workaround is to do something like this:

session.query(table1.metadata.op(':')(sqlalchemy.text('size')))

This is not only ugly but also means that I can't prototype my operations using another database like PostgreSQL before using my code live on snowflake.

This makes JSON access possible using the regular SQLAlchemy syntax.

[u21-yizhang]

I fully support this lol

[masamitsu-murase]

Will this pull request be merged into the main branch?
I would like to know the future plan regarding the support of JSON.

I also developed a small library, snowflake-sqlalchemy-json, to support JSON and func.flatten(...), but I'm not sure whether creating a pull request based on this library will help this project.
If someone knows the future plan about JSON, could you share it with me?

Regards,
Murase