Add keepAlive to `fetch`

[styfle]

Describe the feature you'd like to request

The current fetch polyfill closes the connection after each request but we would like to keep it open.

Describe the solution you'd like

We could utilize a keep-alive connection by assigning in a default Agent and achieve similar performance to undici-fetch.

Describe alternatives you've considered

Swap node-fetch for undici-fetch.

[balazsorban44]

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