[typescript-fetch] Add support for File (application/octet-stream) response types

[spallister]

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Description of the PR

This is a copy of a similar PR I submitted to the swagger-codgen repo here. A while back, @wing328 reached out to let me know the majority of the community moved over here, where I might want to consider re-submitting my PR.

The purpose of this PR is to return application/octet-stream response types as Files, instead of processing them as JSON. In the PR I had submitted to the swagger-codegen repo, I was returning such response types as Blobs, but soon after had a need to obtain the file's name (i.e. which File supports, but Blob does not).

Note that this PR includes a change for DefaultGenerator.java, as I needed to add a property to the mustache template model to determine whether the API contained at least one 'File Response'-type Operation. In particular, this was needed so that I could conditionally include a responseToFile helper method within api.mustache as necessary (i.e. our TS linter would otherwise throw an unused method warning for applications where no 'File Response'-type Operations were in use).

It looks like there is a TypeScript-fetch rewrite taking place over at #569, but figured I'd submit this in case it can be merged in the interim, or incorporated into that rewrite.

Fixes #531

[wing328]

@spallister I'll review this change over the weekend and let you know if I've any question. I may need your help to resolve the merge conflicts.

[macjohnny]

reminder: conflicts should be resolved

[macjohnny]

@spallister @wing328 ping

[wing328]

@spallister can you please resolve the merge conflicts when you've time? then we'll merge it into master if no one has further feedback on this PR.

[deefdragon]

Is this still viable in 5.x/6.0 or have too many changes been made since this was introduced to merge it in?

[wlievens]

Is there any news for this feature? Being able to use the generated API for downloading blobs seems essential.

[TiFu]

@wlievens: This PR would unfortunately need to be updated before it can be accepted given the merge conflicts and submission ~5 years ago (not sure if anything substantially changed that would require changes to this PR's implementation).

If you have some time on your hands, we would of course welcome a PR resolving this :)