[golang] Add support for text parsing with json struct tags

[aburdulescu]

Add struct tags to Go native structs generated when object API is used.

This allows to use the same JSON file as for C++ text parsing(i.e. snake_case vs CamelCase) and thus enabling text parsing for Go(when using object API).

See also issue #7004

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[dbaileychess]

@CasperN Do you know Go?

[dbaileychess]

This LGTM, though I don't know Golang. Since it is a fairly simple change, if it does break people we can revert it easily enough. Thanks!