batches: retry changeset spec upload by default

[LawnGnome]

Fixes https://github.com/sourcegraph/sourcegraph/issues/30431.

This only adds retries to changeset spec upload — although the original ticket also mentions batch spec upload, I'm honestly less concerned about that. Let's deal with the n upload case before the +1 upload case.

There's a little bit of plumbing here to handle the retries, which I think is mostly uninteresting.

Test plan

I've added new unit tests to cover the retrier, CreateChangesetSpec, and tested this quickly by hand to ensure the overall commands still operate.

[chrispine]

This has been sitting here for a while; what's the plan for this? Does it still make sense to merge it?