Provide SDK transform to get a qual value

[e-gineer]

It's common in some tables to need to return unchanged the qual that was passed in. For example, a query column in a search table.

I'd love SDK support to easily return the value of a qual as a column. e.g. return the query string column with the value of the query qual.

Consider this Slack search table as an example:

func tableSlackSearch() *plugin.Table {
	return &plugin.Table{
		Name:        "slack_search",
		Description: "Search slack for anything using a query.",
		List: &plugin.ListConfig{
			Hydrate:    listSearches,
			KeyColumns: plugin.SingleColumn("query"),
		},
		Columns: []*plugin.Column{
			// Top columns
			{Name: "query", Type: proto.ColumnType_STRING, Hydrate: queryString, Transform: transform.FromValue(), Description: "The search query."},
			...
		},
	}
}

func queryString(ctx context.Context, d *plugin.QueryData, _ *plugin.HydrateData) (interface{}, error) {
	quals := d.KeyColumnQuals
	q := quals["query"].GetStringValue()
	return q, nil
}