sql: support @ operator

[otan]

Currently we've special cased the @ operator for column specification, e.g. tbl_name@3.

This special casing prevents us from using @ for legitimate Postgre-esque operations, such as @ for bounding box for PostGIS.

Jira issue: CRDB-3574

[jordanlewis]

We also use it for index specification: SELECT * FROM t@idx WHERE...

I didn't know we used it to specify columns. That usage feels like it could be more easily removed. I think people like the index hints quite a bit, though.

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