Support COPY INTO ... (SELECT fields FROM @stage)

[flaneur2020]

Summary

there's a data load with transformation feature in snowflake:

/* Data load with transformation */
COPY INTO [<namespace>.]<table_name> [ ( <col_name> [ , <col_name> ... ] ) ]
     FROM ( SELECT [<alias>.]$<file_col_num>[.<element>] [ , [<alias>.]$<file_col_num>[.<element>] ... ]
            FROM { internalStage | externalStage } )
[ FILES = ( '<file_name>' [ , '<file_name>' ] [ , ... ] ) ]
[ PATTERN = '<regex_pattern>' ]
[ FILE_FORMAT = ( { FORMAT_NAME = '[<namespace>.]<file_format_name>' |
                    TYPE = { CSV | JSON | AVRO | ORC | PARQUET | XML } [ formatTypeOptions ] } ) ]
[ copyOptions ]

also, we can consider adding transform functions into the SELECT clause, so we can make some simple transforms during the COPY clause, like extract fields from JSON.

COPY INTO books FROM (SELECT parse_json($0).title, parse_json($0).url FROM @stage1)

[BohuTANG]

cc @sundy-li

[ClSlaid]

Is this related to #7211? should wait for #7211 to complete first?

[BohuTANG]

Let's do this simpler task first :)