[typescript] add abort signal to requestcontext

[davidgamero]

major browsers support init.signal parameter, and the whatwg-fetch package supports it and so does the node-fetch package which are the two packages used in generated fetch calls

exposing this signal on the RequestContext will allow users to cancel calls with an abort signal or take control of timeout functionality

clean up tests to use more concise async arrow functions instead of more verbose promise constructor

this implementation only adds the signal support on node

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