Allow clamping of EXEC_TIME_LIMIT_S Query Option per pool

Setting the EXEC_TIME_LIMIT_S (for example to 1800 which equals to 30 minutes execution time) is commonly used to protect the cluster against runaway queries. This is also suggested in the docs:
https://impala.apache.org/docs/build/html/topics/impala_exec_time_limit_s.html

However currently users can disable this feature with explicitly running the set EXEC_TIME_LIMIT_S=0 before their queries. In production environments this typically causes all sorts of problems.
Similarly to the "Clamp MEM_LIMIT Query Option" we could also limit the EXEC_TIME_LIMIT_S to a value set by default on the pool level.

Hive solves similar requirements with "config blacklists" (so some SET statements are restricted), that could be an alternative but probably too restrictive solution.