salesforce: add generic HTTP helpers

Salesforce exposes request which enables a low-level HTTP request to be made to the salesforce API directly.

The old 4.8 salesforce also exported axios directly (maybe common http too?)

The new API does not.

We need to work out how to expose a generic set of HTTP helpers to call external services. It is critical that these helper do not append salesforce credentials

This may be related to the util pattern #522 . We could export standard functions on util.request. Or we could export a http namespace and have operations like http.get. Maybe even both.

A more seamless API would be to expose a single API (request/get/post) use jsforce.request for relative urls, and common.http for any other absolute URL. The worry there is that the return and error handling are likely to be different - how important is that, and can we consolidate?

Start by comparing the old 4.8 Salesforce exports with the new API, then read the related util pattern in issue #522. The issue leaves the API shape undecided between util.request, an http namespace, or shared request/get/post functions; clarify that design and define how credential handling, return values, and errors should work before implementation.