Feature: support basic select at statement

[dantengsky]

Summary:

Enhance the select query, so that a time point can be specified, sth like this

select * from t at(offset => -2) as t where ...

https://docs.snowflake.com/en/sql-reference/constructs/at-before.html

A basic idea of impl (not a strict requirement, any other functionality equivalent impls are ok)

• enhance the corresponding parser components
  encode the timepoint specification (the at(offset...) parts) into DfQueryStatement

• carries the timepoint spec all the way to ToReadDataSourcePlan::read_plan_with_catalog

  • detects if the underlying table instance support time travel.
  • if it does, navigate there, and construct the ReadDataSourcePlan accordingly

  also see Feature: add timestamp field to table snapshot #5514