[SPARK-16579][SPARKR] add install.spark function

## What changes were proposed in this pull request?

Add an install_spark function to the SparkR package. User can run `install_spark()` to install Spark to a local directory within R.

Updates:

Several changes have been made:

- `install.spark()`
    - check existence of tar file in the cache folder, and download only if not found
    - trial priority of mirror_url look-up: user-provided -> preferred mirror site from apache website -> hardcoded backup option
    - use 2.0.0

- `sparkR.session()`
    - can install spark when not found in `SPARK_HOME`

## How was this patch tested?

Manual tests, running the check-cran.sh script added in #14173.

Author: Junyang Qian <junyangq@databricks.com>

Closes #14258 from junyangq/SPARK-16579.