properly handle options.signal passed in by clients

[alexkli]

If a client sets options.signal, and triggers an abort this way, we need to respect this and not continue retrying.

See these contributions from @Ravelle in his fork:

riff-tech#1
riff-tech#2

[Ravelle]

Hey, thanks for the mention! I did consider PRing this fork but I neglected an edge case and didn't have time to fix it:
https://github.com/riff-tech/node-fetch-retry/blob/b9a2e826612e880c8f619ba05bf72e347f9c35a0/index.js#L221
Basically I would prefer that instead of directly overriding the calling appication's onabort handler, we extend it, that way it further avoids colliding with the expected functionality for abort signals.