[Doris On ES] Support fetch `_id` field from ES

[wuyunfeng]

Is your feature request related to a problem? Please describe.
Some DOE(Doris On ES)'s user maybe want to obtain the native ES document _id which is primary-key for ES Index. They give _id a certain business meaning

Describe the solution you'd like

1. Modify Doris FE's verification for column name to support name prefixed by _, introduce _id into ES external Table
2. add _id filter_path for es_scan_reader just only for _source mode, because DOE would not get any field or meta-field value from stored_fields
3. If you encounter _id field just obtain value from top inner-hit node

Describe alternatives you've considered
Do not provide some field-mapping like es-hadoop or presto-es because of a lot of redundant code logic, just explicitly claimed _id column in es table.

Additional context
Please Note: if you needed _id field value, DOE would not enable doc_values scan .