proposal: sqllogictest custom extension

[skyzh]

I'm thinking of using +label after each statement ok and query to do some custom extensions over the original sqllogictest syntax. An example here:

https://github.com/cmu-db/bustub/blob/85477ace4eb3ff6531ccfb075dbc283ff99dbdf1/test/sql/p3.14-topn.slt#L354-L359

What we can do:

• +explain print the query plan
• +repeat:10 repeat for 10 times
• +ensure:plan_node ensure there's some plan node in the plan
• +session:name run this query in a given session, e.g., to test the behavior of txn