### What changes were proposed in this pull request?
This PR extends the RESET command to support reset SQL configuration one by one.
### Why are the changes needed?
Currently, the reset command only supports restore all of the runtime configurations to their defaults. In most cases, users do not want this, but just want to restore one or a small group of settings.
The SET command can work as a workaround for this, but you have to keep the defaults in your mind or by temp variables, which turns out not very convenient to use.
Hive supports this:
https://cwiki.apache.org/confluence/display/Hive/HiveServer2+Clients#HiveServer2Clients-BeelineExample
reset <key> | Resets the value of a particular configuration variable (key) to the default value.Note: If you misspell the variable name, Beeline will not show an error.
-- | --
PostgreSQL supports this too
https://www.postgresql.org/docs/9.1/sql-reset.html
### Does this PR introduce _any_ user-facing change?
yes, reset can restore one configuration now
### How was this patch tested?
add new unit tests.
Closes#29202 from yaooqinn/SPARK-32406.
Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>
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