feat(docuploader): add destination-prefix option

[tbpg]

The prefix option will be used when uploading DocFX YAML. We need to
distinguish HTML docs vs DocFX YAML. Then, another job will convert the
YAML to HTML. This way, all DocFX tarballs can start with docfx- and we can
strip that prefix when uploading the resulting HTML.

[tmatsuo]

I'm not sure if it's a good idea to have this option.

Instead can we always upload html files?

[JustinBeckwith]

I love this idea. One of the original goals was to try getting to an intermediary format that could be re-generated into HTML as the template on cloudsite changes. By storing this in YAML instead of HTML, we can re-generate any time we want with different visuals.

[tmatsuo]

I'm not opposed to have a capability of only re-generating the HTML from yaml files.

I don't feel quite right to have this option and metadata.

How about just to have --destination-prefix option?

[tmatsuo]

I prefer --destination-prefix option

[tmatsuo]

I may miss the big picture, so I could be convinced if there's a reason to add format metadata.

[tbpg]

I switched it to --destination-prefix.

The risk of this is that each client library will be responsible for setting the right docfx prefix. Typos will not be caught when calling the tool -- only when docs don't show up. We also need to be consistent in how the prefix is applied, so we can remove it in the HTML generation job.