Add proxy support for global fetch

[Kruspe]

Issue #650:

Description of changes:
Support setting the dispatcher for the global fetch implementation.
Instead of ignoring the created request, we now use it and pass the dispatcher option if it is available.

Added an integration test with a reverse proxy to check that the trace header is there.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Kruspe]

My first idea was to pass down the entire args again but this would overwrite the changes we do to the request like adding the trace header as the headers from the options take priority over the headers that are already set for a request.

[berndfuhrmann]

Consider this code:

const agent = new Agent({maxSockets: 1234});
const request = new Request('someurl', { dispatcher: agent });
fetch(request);

Here, fetch is only called with one parameter. The dispatcher is "baked in" into the request. For undici, it's actually a symbol property. When you call baseFetchFunction(requestClone, fetchOptions), requestClone doesn't contain that property, So that per-request-setting wouldn't be passed on.

[berndfuhrmann]

Made an attempt to fix this. Sorry for the complexity, but it should work more reliably for you.
#698

[Kruspe]

#698 fixes a different issue that is there so I would open another issue to point to your PR to.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.56%. Comparing base (3b89008) to head (60c56b3).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #696   +/-   ##
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  Coverage   83.56%   83.56%           
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  Files          36       36           
  Lines        1813     1813           
=======================================
  Hits         1515     1515           
  Misses        298      298           

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