[SPARK-3696]Do not override the user-difined conf_dir

https://issues.apache.org/jira/browse/SPARK-3696

We see if SPARK_CONF_DIR is already defined before assignment.

Author: WangTaoTheTonic <barneystinson@aliyun.com>

Closes #2541 from WangTaoTheTonic/confdir and squashes the following commits:

c3f31e0 [WangTaoTheTonic] Do not override the user-difined conf_dir
[SPARK-2693][SQL] Supported for UDAF Hive Aggregates like PERCENTILE

Implemented UDAF Hive aggregates by adding wrapper to Spark Hive.

Author: ravipesala <ravindra.pesala@huawei.com>

Closes #2620 from ravipesala/SPARK-2693 and squashes the following commits:

a8df326 [ravipesala] Removed resolver from constructor arguments
caf25c6 [ravipesala] Fixed style issues
5786200 [ravipesala] Supported for UDAF Hive Aggregates like PERCENTILE