[SPARK-14452][SQL] Explicit APIs in Scala for specifying encoders

## What changes were proposed in this pull request?
The Scala Dataset public API currently only allows users to specify encoders through SQLContext.implicits. This is OK but sometimes people want to explicitly get encoders without a SQLContext (e.g. Aggregator implementations). This patch adds public APIs to Encoders class for getting Scala encoders.

## How was this patch tested?
None - I will update test cases once https://github.com/apache/spark/pull/12231 is merged.

Author: Reynold Xin <rxin@databricks.com>

Closes #12232 from rxin/SPARK-14452.