This modifies the high level rest client to allow calling code to
customize per request options for the bulk API. You do the actual
customization by passing a `RequestOptions` object to the API call
which is set on the `Request` that is generated by the high level
client. It also makes the `RequestOptions` a thing in the low level
rest client. For now that just means you use it to customize the
headers and the `httpAsyncResponseConsumerFactory` and we'll add
node selectors and per request timeouts in a follow up.
I only implemented this on the bulk API because it is the first one
in the list alphabetically and I wanted to keep the change small
enough to review. I'll convert the remaining APIs in a followup.
b225f5e5c6