Add "stub_distribution" and "upload" metadata fields

[srittau]

These optional fields allow more control over what gets uploaded from the typeshed side:

• stub_distribution allows to change the uploaded distribution name.
• Setting upload to false prevents uploading a package (e.g. in case of errors).

[srittau]

I believe that these fields could prove useful in the future, not matter what we decide to do for python/typeshed#9246.