Extend the SparkContextOptions

Currently, it's possible to use the `SparkContextOptions` to "pass" a custom `SparkContext` to the Spark runner.

However, more than constructing a complete `SparkContext` outside of the runner, an user may want to "tweak" the context created internally by the runner.
We could "extend" the `SparkContextOptions` to pass additional options used to construct and populate the `JavaSparkContext` like security, dependencies management, additional properties.

Imported from Jira [BEAM-2381](https://issues.apache.org/jira/browse/BEAM-2381). Original Jira may contain additional context.
Reported by: jbonofre.

Start by locating SparkContextOptions and the Spark runner, then trace how the runner constructs and populates JavaSparkContext. Determine how security, dependency-management, and additional properties should be represented and passed through; done means callers can tweak the internally created context without constructing a complete SparkContext themselves.

Written by the indexing model from the issue text.