Add Function to Obtain Inner Map Schemata

This function allows you to drill down into the schema of maps. The REST API and JDBC interfaces will only return `MAP`, `LIST` for the MAP, however, it is not possible to get
the schema of the inner map. The function `getMapSchema(<MAP>)` will return a `MAP` of the fields and datatypes.

1. 1. 1. Example Usage

Using the data below, the query below will return the schema as shown below.
```bash
apache drill> SELECT getMapSchema(record) AS schema FROM dfs.test.`schema_test.json`;
----------------------------------------------------------------------------------

----------------------------------------------------------------------------------

----------------------------------------------------------------------------------
1 row selected (0.298 seconds)
```

```json
{
"record" : {
"int_field": 1,
"double_field": 2.0,
"string_field": "My string",
"int_list": [1,2,3],
"double_list": [1.0,2.0,3.0],
"map":

{ "nested_int_field" : 5, "nested_double_field": 5.0, "nested_string_field": "5.0" }

},
"single_field": 10
}
```

The function returns an empty map if the row is `null`.