[Go] File Download Fix Return Type

[icubbon]

Addresses #11965
The Go client.decode expected a **os.File but the return type is actually a *os.File.

This is almost a copy/pasta of #12685 but that PR has been sitting idle for 5 months without passing all of the checks to be merged in.

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[wing328]

@icubbon thanks for the fix. Let's give it a try.

[wing328]

Some indentions in this files and others are using 4-space instead of tabs. I'll file another PR to update that.

[wing328]

UPDATE: filed #14189

[wing328]

@icubbon shall we keep the mapping (pointer to os.File) and fix the code to properly return a pointer to os.File instead?

[wing328]

cc @DouglasDwyer (the author of #12685)

[wing328]

I'm afraid we need to revert this change and instead fix the endpoint to properly return a *os.File instead.

[icubbon]

I can take a stab at fixing the mapping to be a *os.File. It makes sense to have it be an *os.File as with the newer version of the generator, complex items are now pointers.

[icubbon]

@wing328 Please take a look at #14340. This reverts the mapping to *os.File and keeps us from having any goofy **os.File references.