Look into conditionally using native fetch when we move to Node 20

[sofisl]

see b/337041278

Once we move to Node 18, we should look into checking whether we can use native fetch instead of node-fetch.

[danielbankhead]

I’ve completed some research on the topic - see:

• https://github.com/googleapis/gaxios/issues/625
• https://github.com/googleapis/gaxios/pull/618

[sofisl]

Awesome, thanks for doing that digging! We can use it to replace node-fetch here, as well.