`buildkite-agent pipeline upload` should chunk uploads for the user

**Is your feature request related to a problem? Please describe.**
We regularly upload pipelines with step quantities > 500. It is frustrating to have to do the chunking as the user.

**Describe the solution you'd like**
`buildkite-agent pipeline upload` should chunk the steps for me, especially if the stdin stream can be made to support it.

**Describe alternatives you've considered**
GNU `parallel` does something like this but it is a papercut.