[SPARK-4574][SQL] Adding support for defining schema in foreign DDL commands.

Adding support for defining schema in foreign DDL commands. Now foreign DDL support commands like:
```
CREATE TEMPORARY TABLE avroTable
USING org.apache.spark.sql.avro
OPTIONS (path "../hive/src/test/resources/data/files/episodes.avro")
```
With this PR user can define schema instead of infer from file, so  support ddl command as follows:
```
CREATE TEMPORARY TABLE avroTable(a int, b string)
USING org.apache.spark.sql.avro
OPTIONS (path "../hive/src/test/resources/data/files/episodes.avro")
```
instead of `_schema`, how about `userSpecifiedSchema`?