spanner: third party dependency exception for github.com/json-iterator/go

[noahdietz]

Module: spanner Usage(s): spanner/value.go

For the time being, it will be exempt from the third party dep check, but please do one of the following:

A. Remove the dependency by switching to a package own directly by Google, handwriting necessary functionality, or using stdlib B. Justify the exception and ack the risks, maintaining the exception indefinitely

[egonelbre]

There is a significant concern from my point of view with json-iterator dependency due to it's heavy reliance on unsafe and modern-go/reflect2. See https://github.com/golang/go/issues/54766#issuecomment-1983808590 for issues that happen from it -- the lucky case is when your compilation or tests fail.

[rahul2393]

Thanks @egonelbre for pointing this out, bumping the priority for now.