[SPARK-8867][SQL] Support list / describe function usage

As Hive does, we need to list all of the registered UDF and its usage for user.

We add the annotation to describe a UDF, so we can get the literal description info while registering the UDF.
e.g.
```scala
ExpressionDescription(
    usage = "_FUNC_(expr) - Returns the absolute value of the numeric value",
    extended = """> SELECT _FUNC_('-1')
                  1""")
 case class Abs(child: Expression) extends UnaryArithmetic {
...
```

Author: Cheng Hao <hao.cheng@intel.com>

Closes #7259 from chenghao-intel/desc_function and squashes the following commits:

cf29bba [Cheng Hao] fixing the code style issue
5193855 [Cheng Hao] Add more powerful parser for show functions
c645a6b [Cheng Hao] fix bug in unit test
78d40f1 [Cheng Hao] update the padding issue for usage
48ee4b3 [Cheng Hao] update as feedback
70eb4e9 [Cheng Hao] add show/describe function support