better representation for numeric sqs-message-attributes

This issue is a carry over from the Lambda runtime project
https://github.com/swift-server/swift-aws-lambda-runtime/issues/53

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https://github.com/swift-server/swift-aws-lambda-runtime/pull/46/files#diff-46c15e7ad33f3c55f1eff630cb7c7d6aR30

https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-message-attributes.html under "Message Attribute Data Types"

Number attributes can store positive or negative numerical values. A number can have up to 38 digits of precision, and it can be between 10^-128 and 10^+126.

right now using string as a stop gap, could use Foundation's Decimal or come up with our own BigNumber abstraction

Start by reviewing the carried-over Lambda runtime issue, the referenced pull request diff, and AWS's SQS Message Attribute Data Types documentation. Determine the required representation for positive and negative numbers with up to 38 digits of precision and the documented exponent range; done means numeric attributes no longer rely on the current String stopgap.

Written by the indexing model from the issue text.