Split upload of large pipelines into multiple requests so that we don't hit Buildkite API limits

[ivy]

Buildkite imposes a default limit of 500 jobs in a single buildkite-agent pipeline upload invocation. We should automatically break up the pipeline for large batches.

[technicalpickles]

We chatted about this one the other day. The 500 job limit is what Buildkite defaults to, but we requested they increase it to 600.

I think if we wanted to increase parallelism of rspec past 600, I don't know if Knapsack Pro is able to handle jobs split across pipelines. We could at least split rspec and graphql into their own pipelines though.

[ngan]

This is done with detached groups.