## What changes were proposed in this pull request?
We allow users to specify hints (currently only "broadcast" is supported) in SQL and DataFrame. However, while SQL has a standard hint format (/*+ ... */), DataFrame doesn't have one and sometimes users are confused that they can't find how to apply a broadcast hint. This ticket adds a generic hint function on DataFrame that allows using the same hint on DataFrames as well as SQL.

As an example, after this patch, the following will apply a broadcast hint on a DataFrame using the new hint function:

```
df1.join(df2.hint("broadcast"))
```

## How was this patch tested?
Added a test case in DataFrameJoinSuite.

Author: Reynold Xin <rxin@databricks.com>

Closes #17839 from rxin/SPARK-20576.