[SPARK-18086] Add support for Hive session vars.

## What changes were proposed in this pull request?

This adds support for Hive variables:

* Makes values set via `spark-sql --hivevar name=value` accessible
* Adds `getHiveVar` and `setHiveVar` to the `HiveClient` interface
* Adds a SessionVariables trait for sessions like Hive that support variables (including Hive vars)
* Adds SessionVariables support to variable substitution
* Adds SessionVariables support to the SET command

## How was this patch tested?

* Adds a test to all supported Hive versions for accessing Hive variables
* Adds HiveVariableSubstitutionSuite

Author: Ryan Blue <blue@apache.org>

Closes #15738 from rdblue/SPARK-18086-add-hivevar-support.