[BUG] Create skipping index failed on nested field of struct type

[dai-chen]

What is the bug?

Create skipping index statement failed using nested field of a struct type as indexed column. See example below.

How can one reproduce the bug?

CREATE TABLE waf_logs
`httpRequest` struct <
                    clientIp: string> ...

CREATE SKIPPING INDEX
ON waf_logs (
  `httpRequest.clientIp` VALUE_SET
)

FlintSparkIndexBuilder.findColumn complains that field httpRequest.clientIp cannot be found.

What is the expected behavior?

Skipping index is created successfully with nested field like httpRequest.clientIp and the values can be refreshed to OpenSearch index without issue.