Scan enums as strings

[k-anshul]

This PR adds ability to scan duckDB enum types as strings by default:

1. This is similar to how julia pkg for duckDB handles enums : https://github.com/duckdb/duckdb/blob/958f1898805822793fefef2b9a0c0e953c1f9fd0/tools/juliapkg/src/result.jl#L126
2. This is also similar to how other go SQL drivers like pgx handle enums out of box :

package main

import (
	"context"
	"database/sql"
	"fmt"
	"log"

	_ "github.com/jackc/pgx/v5/stdlib"
)

// Define a custom type for the enum
type Status string

const (
	Pending  Status = "pending"
	Approved Status = "approved"
	Rejected Status = "rejected"
)

func main() {

	// Connect to the PostgreSQL database using pgxpool
	connString := "postgresql://postgres:postgres@localhost:5432/postgres"
	conn, err := sql.Open("pgx", connString)
	if err != nil {
		log.Fatal(err)
	}
	defer conn.Close()

	// Create the table with an enum column
	_, err = conn.ExecContext(context.Background(), `
		CREATE TYPE status_enum AS ENUM ('pending', 'approved', 'rejected');
		CREATE TABLE your_table (
			id SERIAL PRIMARY KEY,
			name VARCHAR(50),
			status status_enum
		);
	`)
	if err != nil {
		log.Fatal(err)
	}

	// Insert some data into the table
	_, err = conn.ExecContext(context.Background(), "INSERT INTO your_table (name, status) VALUES ($1, $2)", "Item 1", Pending)
	if err != nil {
		log.Fatal(err)
	}

	// Query the data and scan the results
	rows, err := conn.QueryContext(context.Background(), "SELECT id, name, status FROM your_table")
	if err != nil {
		log.Fatal(err)
	}
	defer rows.Close()

	for rows.Next() {
		var id int
		var name string
		var status Status

		err := rows.Scan(&id, &name, &status)
		if err != nil {
			log.Fatal(err)
		}

		fmt.Printf("ID: %d, Name: %s, Status: %s\n", id, name, status)
	}

	if err := rows.Err(); err != nil {
		log.Fatal(err)
	}
}

[marcboeker]

Great addition, thanks @k-anshul. I've refactored the tests a little bit.
Could you please provide a reference or documentation link for further understanding of the following part?

internalType := C.duckdb_enum_internal_type(ty)
	switch internalType {
	case C.DUCKDB_TYPE_UTINYINT:
		idx = uint64(get[uint8](vector, rowIdx))
	case C.DUCKDB_TYPE_USMALLINT:
		idx = uint64(get[uint16](vector, rowIdx))
	case C.DUCKDB_TYPE_UINTEGER:
		idx = uint64(get[uint32](vector, rowIdx))
	case C.DUCKDB_TYPE_UBIGINT:
		idx = get[uint64](vector, rowIdx)
	default:
		return "", errInvalidType
	}

[k-anshul]

Actually I couldn't find any documentation. I looked at the test case around enums in the CAPI src code here and then modified it as per how we scan decimal in this driver.