[typescript-fetch] Allow fetch to be passed in configuration

[djMax]

When generating typescript-fetch code, I would like to be able to pass in a single argument to the various API constructors/methods that covered all the options, INCLUDING the fetch function. Our use case is to automatically inject various metrics and headers to outgoing requests and incoming responses, and they are "per instance." So overriding fetch is great, BUT, it means if I want to create a client with these overrides, I need to do:

new PetsApi(...myCustomSettings)

That function takes three arguments now - config, fetch, and basePath. It already allows basePath to be passed into the config, so I'm suggesting that fetch is also allowed to be passed into the config. PR forthcoming if I can grok it.

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[djMax]

Never mind, I see that 4.0 is a major refactor that has middleware that likely does exactly this. Still trying to read through it and then try it to verify, but guidance appreciated.