What is this feature?

Return the SQL schema for all DS queries in request (to provide information to AI / Autocomplete for SQL expressions).

All DS queries are treated as if they were inputs to SQL expressions in terms of conversion, regardless if they are selected in a query or not.

It takes a regular expressions requests, filters it to DS queries, and returns a schema object. (TODO: not hooked up to endpoint yet, and schema DTO (map[refID][]Column) not worked out exactly yet)

Why do we need this feature?

[Add a description of the problem the feature is trying to solve.]

Who is this feature for?

[Add information on what kind of user the feature is for.]

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Output will be like

{
  "genres": [
    {
      "Name": "asin",
      "MySQLType": "text",
      "Nullable": true,
      "DataFrameFieldType": "*string"
    },
    {
      "Name": "genre",
      "MySQLType": "text",
      "Nullable": true,
      "DataFrameFieldType": "*string"
    }
  ],
  "main": [
    {
      "Name": "asin",
      "MySQLType": "text",
      "Nullable": true,
      "DataFrameFieldType": "*string"
    },
    {
      "Name": "author",
      "MySQLType": "text",
      "Nullable": true,
      "DataFrameFieldType": "*string"
    },
    {
      "Name": "copyright",
      "MySQLType": "double",
      "Nullable": true,
      "DataFrameFieldType": "*float64"
    },
    {
      "Name": "description",
      "MySQLType": "text",
      "Nullable": true,
      "DataFrameFieldType": "*string"
    },
    {
      "Name": "formatType",
      "MySQLType": "text",
      "Nullable": true,
      "DataFrameFieldType": "*string"
    }
  ]
}

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.