Use keep-alive in all SDK HTTP requests

[0div]

Description

This is the client side changes needed to make sure keep-alive is consistent across all server-side requests.

Test

For now, making sure all legacy tests pass, will validate that the header is present server-side.

[linear]

E2B-2185 Ensure we are using keepalives in our SDKs

[changeset-bot]

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Latest commit: 38e06c3

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[0div]

@jakubno , is this what you envisioned wrt https://linear.app/e2b/issue/E2B-2185/ensure-we-are-using-keepalives-in-our-sdks ? If so, ill move forward with the API/envd part.

[ValentaTomas]

If we are adding the keepalive we might revisit the max connection limits of the clients/fetch.