[SPARK-13263][SQL] SQL Generation Support for Tablesample

In the parser, tableSample clause is part of tableSource.
```
tableSource
init { gParent.pushMsg("table source", state); }
after { gParent.popMsg(state); }
    : tabname=tableName
    ((tableProperties) => props=tableProperties)?
    ((tableSample) => ts=tableSample)?
    ((KW_AS) => (KW_AS alias=Identifier)
    |
    (Identifier) => (alias=Identifier))?
    -> ^(TOK_TABREF $tabname $props? $ts? $alias?)
    ;
```

Two typical query samples using TABLESAMPLE are:
```
    "SELECT s.id FROM t0 TABLESAMPLE(10 PERCENT) s"
    "SELECT * FROM t0 TABLESAMPLE(0.1 PERCENT)"
```

FYI, the logical plan of a TABLESAMPLE query:
```
sql("SELECT * FROM t0 TABLESAMPLE(0.1 PERCENT)").explain(true)

== Analyzed Logical Plan ==
id: bigint
Project [id#16L]
+- Sample 0.0, 0.001, false, 381
   +- Subquery t0
      +- Relation[id#16L] ParquetRelation
```

Thanks! cc liancheng

Author: gatorsmile <gatorsmile@gmail.com>
Author: xiaoli <lixiao1983@gmail.com>
Author: Xiao Li <xiaoli@Xiaos-MacBook-Pro.local>

This patch had conflicts when merged, resolved by
Committer: Cheng Lian <lian@databricks.com>

Closes #11148 from gatorsmile/tablesplitsample.