[SPARK-21519][SQL] Add an option to the JDBC data source to initialize the target DB environment

Add an option to the JDBC data source to initialize the environment of the remote database session

## What changes were proposed in this pull request?

This proposes an option to the JDBC datasource, tentatively called " sessionInitStatement" to implement the functionality of session initialization present for example in the Sqoop connector for Oracle (see https://sqoop.apache.org/docs/1.4.6/SqoopUserGuide.html#_oraoop_oracle_session_initialization_statements ) . After each database session is opened to the remote DB, and before starting to read data, this option executes a custom SQL statement (or a PL/SQL block in the case of Oracle).

See also https://issues.apache.org/jira/browse/SPARK-21519

## How was this patch tested?

Manually tested using Spark SQL data source and Oracle JDBC

Author: LucaCanali <luca.canali@cern.ch>

Closes #18724 from LucaCanali/JDBC_datasource_sessionInitStatement.