[Feature request] Expose CookieAccessInfo from cookiejar in Node

In Node, we have access to an `Agent`'s cookie jar on the `.jar` property, but in order to retrieve cookies from it (useful in certain cases including testing) [we also need access to `CookieAccessInfo`](https://github.com/bmeck/node-cookiejar#methods-1), which currently isn't exposed by Superagent.

Currently, the way to work around this would be to import `cookiejar` in our own code, however:

1. Having an implicit dependency is a bad idea, not least because there's no guarantee that we'll get the version of `cookiejar` that Superagent is using. A different version is not guaranteed to be compatible, and at the very least it would break `CookieAccessInfo.All` because it's [used like a symbol](https://github.com/bmeck/node-cookiejar/blob/7d0d6317f5cdecca7fa1eebd1126eeb40e56e2cc/cookiejar.js#L133).

2. If we make the dependency explicit in `package.json` we have to take care to match the version that Superagent uses, which is annoying and brittle - even then there may not be a strong guarantee that Superagent uses the version we import.

For these reasons, could `CookieAccessInfo` (or alternatively, the whole `cookiejar` module) be exposed somewhere? Maybe as a static property of `Agent`? Then we could do `superagent.agent.CookieAccessInfo`. Or maybe as a prototype property? Another option is to expose it on the main import, but I'm not sure if that makes sense or not. Or even just a separate file we could import like `'superagent/cookiejar'`. If we can work out which one is preferred I'd be happy to throw up a PR myself. I'd also like it to be exposed from Supertest, if that makes any difference.

Previous request from a long time ago that was closed without explanation: #192

Trace Agent's .jar property and the bundled cookiejar dependency first; compare exposing CookieAccessInfo on Agent, the main import, or a subpath such as superagent/cookiejar. Done means a stable Node-facing export that avoids consumers importing cookiejar directly, with Supertest coverage if that remains in scope.

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