Migrate to internal node implementation of fetch

[brendandburns]

We are going to remove the node-fetch dependency and move to node's internal implementation.

[cjihrig]

I think this might also resolve #2127 since native fetch has a keepalive option, which node-fetch does not.

EDIT: This is probably wrong. The keepalive in the fetch spec is something different.

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[mstruebing]

/remove-lifecycle stale

[k8s-triage-robot]

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/lifecycle stale

[mstruebing]

/remove-lifecycle stale

[davidgamero]

Upstream PR on the generator has merged to use undici.
OpenAPITools/openapi-generator#22408

It relies on the "undici" module instead of using node-provided undici as features like interceptors, middleware, and stream are not yet available in the node native version.
https://undici.nodejs.org/#/?id=undici-module

I tested locally regenerating the client. As expected, most of the types in Config that rely on https/node-fetch are subtly different than the undici interfaces and will require updates.

I havent tested the mocking yet. As previously discussed, nock will no longer work for our tests with undici.

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/lifecycle stale