Optionally prepend schema for multi-tenancy

[nicksan222]

This PR adds the flag emit_schema_name.
This will put before all the tables a %s, and will replace them inside the function

The generated result would look something like this

const getItemsByIds = `-- name: GetItemsByIds :many
SELECT id FROM ` + "`" + `%s` + "`" + `.items
ORDER BY ` + "`" + `index` + "`" + ` ASC
`

// Will add schema name as parameter
func (q *Queries) GetItemsByIds(ctx context.Context, schema string) ([]uint32, error) {
        // Will replace at runtime the schema placeholder
	rows, err := q.db.QueryContext(ctx, strings.ReplaceAll(getItemsByIds, "%s", schema))
	if err != nil {
		return nil, err
	}
	defer rows.Close()
	var items []uint32
	for rows.Next() {
		var id uint32
		if err := rows.Scan(&id); err != nil {
			return nil, err
		}
		items = append(items, id)
	}
	if err := rows.Close(); err != nil {
		return nil, err
	}
	if err := rows.Err(); err != nil {
		return nil, err
	}
	return items, nil
}