Support returning column names of prepared statement

[PThorpe92]

I'm in the process of writing the Go database/sql driver, and this requires that the names of the columns are able to be queried from a prepared statement.

database/sql/driver

type StmtQueryContext interface {
	// ...
	QueryContext(ctx context.Context, args []NamedValue) (Rows, error)
}

We need to be able to call stmt.columns() to return an array of strings of the column names of the query.

type Rows interface {
	// Columns returns the names of the columns.
	Columns() []string
     // ...
}