internal/v4: implement archive uploading

We don't yet verify bundles against charms in the store.

This required some refactoring of the testing code;
the most important change is AssertJSONCall actually
makes a real HTTP call through a socket, as that
allows us to correctly test the ContentLength logic.
It also gives us greater assurance that the logic
will work over a real HTTP connection. The overhead
seems to be negligible.
Yes, but it's arguably not right to use it because we're not creating it the usual way.
It's not a big issue, but is probably worth fixing at some point.