feat: support scanning from big.Int values

[LuccaBitfly]

This PR adds scanning big.Int values into decimal.Decimal.

The clickhouse-go driver stores certain column types like Int128 as big.Int. Currently, the decimal lib (v1.4.0) returns the error could not convert value '{neg:false abs:[7345112218517]}' to byte array of type 'big.Int' when trying to scan such a column into a decimal.Decimal.

Without this PR, I need a workaround like:

type MyDecimal decimal.Decimal

func (d *MyDecimal) Scan(value any) error {
	switch v := value.(type) {
	case big.Int:
		*d = MyDecimal(decimal.NewFromBigInt(&v, 0))
		return nil
	default:
		return (*decimal.Decimal)(d).Scan(value)
	}
}