Add spec for `Long` custom scalar type

[jakobmerrild]

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The Long data type is commonly used in many applications because the largest value that can be held in an Int value is 2^31-1. See: https://github.com/graphql/graphql-spec/issues/73

I'm honestly torn about whether or not output strings should be allowed 👀 . As discussed in the linked issue common clients (JavaScript) does not read JSON numbers above 2^53-1 into number without loss of precision.

Also accepting String inputs without accepting String outputs seem inconsistent.

[linux-foundation-easycla[bot]]

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