support rollapp tokens with different token decimals

[mtsitrin]

currently we enfroce 18 decimals

func (dm DenomMetadata) Validate() error {
...
	// validate exponent
	// TODO: currently only 18 decimals are supported 
	if AllowedDecimals(dm.Exponent) != Decimals18 {
		return fmt.Errorf("invalid exponent")
	}

	return nil
}

when changed, need to handle IRO support as well

func (lbc BondingCurve) SupplyDecimals() int64 {
	// TODO: allow to be set on creation instead of using default
	rollappTokenDefaultDecimals := int64(18)
	return rollappTokenDefaultDecimals
}