Stacked review changes for #3996

[moskyb]

Stacked on #3996

This PR suggests a couple of changes to #3996:

• Split job api promise failure logic into HTTP stuff (which the handler handles) and coordination (a small, dedicated coordinator object)
• Job API HTTP errors are now reserved for errors in the job API itself — a 5xx response from the Buildkite Agent API no longer returns an error (though still has to be handled by the caller
• Makes it so that the HTTP client constructed for the job API to call the agent API retains settings like NO_HTTP2, has the right user agent, etc