Adds a data source to fetch the most recently published Spot Price
value for a given EC2 instance type and availability zone. The value
can be manipulated and fed into spot price parameters in other resources,
allowing for dynamic adjusted of spot requests.

since we are only interested in fetching the latest SpotPrice,
we don't need to iterate over a series of requests, as we only
ever need to fetch call's worth of data

add a test for sourcing an aws_ec2_spot_price data source using
filter{} blocks exclusively. this commit also involves a rework of
the DescribeSpotPriceHistory AWS API call, namely using the native
SDK paging function; without this function, making a singular direct
call to the DescribeSpotPriceHistory SDK function returned a response with
an empty slice, but the underlying API call returned a response with a non-nil
NextToken value. to support this, we simply use the
DescribeSpotPriceHistoryPages unconditionally during reads, and
add all found SpotPriceHistory objects to a slice.