[Doris On ES] Support fetch _id field from ES (#3900)
More information can be found: https://github.com/apache/incubator-doris/issues/3901
The created ES external Table must contains `_id` column if you want to fetch the Elasticsearch document `_id`.
```
CREATE EXTERNAL TABLE `doe_id2` (
`_id` varchar COMMENT "",
`city` varchar COMMENT ""
) ENGINE=ELASTICSEARCH
PROPERTIES (
"hosts" = "http://10.74.167.16:8200",
"user" = "root",
"password" = "root",
"index" = "doe",
"type" = "doc",
"version" = "6.5.3",
"enable_docvalue_scan" = "true",
"transport" = "http"
);
Query:
```
mysql> select * from doe_id2 limit 10;
+----------------------+------+
| _id | city |
+----------------------+------+
| iRHNc3IB8XwmcbhB7lEB | gz |
| jBHNc3IB8XwmcbhB71Ef | gz |
| jRHNc3IB8XwmcbhB71GI | gz |
| jhHNc3IB8XwmcbhB71Hx | gz |
| ThHNc3IB8XwmcbhBkFHB | sh |
| TxHNc3IB8XwmcbhBkFH9 | sh |
| URHNc3IB8XwmcbhBklFA | sh |
| ahHNc3IB8XwmcbhBxlFq | gz |
| axHNc3IB8XwmcbhBxlHw | gz |
| bxHNc3IB8XwmcbhByVFO | gz |
+----------------------+------+
```
NOTICE:
This change the column name format to support column name start with "_".
355df127b7