Make HTTP headers configurable

[livthomas]

We want to use our backend as a proxy to Segment API in order to bypass ad-blockers. We have CSRF protection in place so all requests to our endpoints need to contain X-CSRF-TOKEN header. But @segment/analytics-next library doesn't enable us to set custom HTTP headers.

It looks like headers are hardcoded in fetch-dispatcher and it's been like in a predecessor of this library (see segmentio/analytics.js-integrations#560).

Can you add a new configuration property which will allow us to append additional HTTP headers? Other people are asking for it as well, see #312 (comment).