add chromedp.NewRemoteAllocatorWithHeader

[jscheithe]

opening this PR just for discussing the changes, not intending to merge this.

In this branch, the wsHeader is passed via parameters all the way down to the DialContext function.
The header can be set like this:
actx, acancel := chromedp.NewRemoteAllocator(ctx, wsUrl, wsHeader)

No public functions are changed, instead XxxWithHeader functions are added where needed.

Pros

• easier to use (no need for a BrowserOption etc.)
• straightforward implementation

Cons ?

This is the branch that is currently used by github.com/contextflow/cf-dicom-report-service.

Todo:

• Unit Tests
• Documentation
• Opening upstream PR