Fix `UpdateOrderParams` for allow updating dollar amount values to zero

[BrianLeishman]

Relevant issue: https://github.com/taxjar/taxjar-go/issues/13

This makes it possible to update dollar amounts of an order/transaction to zero.

⚠️ This is a breaking change! ⚠️

Normally I wouldn't request a breaking change at all, and this case we could get around that by create a new struct with the pointer versions and allowing someone to use either, but I feel since this is a bug, it's worth forcing the fix, otherwise (like in our case) failed transaction updates will go completely unnoticed.

I also added two helper functions, pulled straight from the AWS source for the same purpose:

https://github.com/aws/aws-sdk-go/blob/main/aws/convert_types.go

// Float64 returns a pointer to the float64 value passed in.
func Float64(v float64) *float64 {
	return &v
}

// Float64Value returns the value of the float64 pointer passed in or
// 0 if the pointer is nil.
func Float64Value(v *float64) float64 {
	if v != nil {
		return *v
	}
	return 0
}

These helper functions make it easier to switch between the pointer of a type and the value of the type, which looks like this (from the updated example file)

res5, err := client.UpdateOrder(taxjar.UpdateOrderParams{
	TransactionID: "13579-246810",
	Amount:        taxjar.Float64(152.72),
	Shipping:      taxjar.Float64(10),
	SalesTax:      taxjar.Float64(10.74),
	LineItems: []taxjar.OrderLineItem{
		{
			ID:                "1",
			Quantity:          1,
			ProductIdentifier: "12-34243-9",
			Description:       "Fuzzy Sweater",
			ProductTaxCode:    "20010",
			UnitPrice:         36.72,
			Discount:          5.51,
			SalesTax:          0,
		},
		{
			ID:                "2",
			Quantity:          1,
			ProductIdentifier: "12-34245-8",
			Description:       "TaxJar Designer T-shirt",
			ProductTaxCode:    "20010",
			UnitPrice:         111,
			SalesTax:          9.85,
		},
	},
})

This was tested in production via my fork already and I was able to confirm this fixed our issue.