[Feature][Subtask] Determine if cli-based uploads are in-scope for alpine publish

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As part of #144, it may be nice to have automated, cli-based upload of publish'd instance archives.

This way, e.g. alpine publish -u <instance name> could automatically upload the .tar.gz[.age] file to some web server and then add the link to it to an index we control.

Before the above is possible, we have to determine:

• If it's appropriate for alpine to automatically upload things
• What storage backend to use (sustainable, not in violation of hosting policies, etc)
• How to programmatically interface with that storage backend

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Blocking on #145

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Pros:

• seamless experience
• sharing & potential for community

Cons:

• Bloat / feature creep / scope creep

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Let's decide that this is probably scope-creep and leave it off. Users can publish their instances and upload them themselves, for later import with the -u option described in #145