fix(fdc): add support Int64 to nativeFromJson

[slightfoot]

Description

If you used a Int64 type in the GraphQL schema it cannot be decoded in the generated code. Int64 is treated as a BigInt in Postgres/DataConnect and as such transmitted through gRPC as a String. The firebase_data_connect package throws This type is unimplemented: Type... because the nativeFromJson function does not support input being a String and the output type being an int.

This PR also fixes the incorrect Type display in the error message for the exception.

@maneesht this one's for you.

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