Support the Bun.js Runtime

[wmluke]

I would be nice to support the bun.js runtime, but we'll first need to wait for these bun.js issues to be addressed:

• undici.Agent is missing async close() method oven-sh/bun#14498
• Support Vitest oven-sh/bun#4145

[RainyPixel]

+1

[ndeloof]

main blocker AFAICT is oven-sh/bun#4474
as we rely on passing a custom Dispatcher to manage the underlying socket

[ndeloof]

it seems bun just ignores undici agent (oven-sh/bun#10642) which we rely on to support various transport protocols. Support for unix domain socket is available using non-standard 🤨 RequestInit.unix attribute (https://bun.com/docs/guides/http/fetch-unix) and I can get most tests (see next) to pass using this hack. We could also pass custom TLS certificates (https://bun.com/docs/runtime/networking/fetch#tls). We would still miss support for windows npipe and ssh. Sounds like a hack anyway, not sane foundations to work with IMHO 😖

bun overrides undici dependency with a custom implementation 🤪, basically just forwarding to the internal fetch API which is missing upgrade support:

error: This function is not yet implemented in Bun
      at notImplemented (undici:13:14)
      at upgrade (undici:124:17)

this is not strictly needed as engine will accept a plain POST request, but could be required by proxies in some context

[ndeloof]

tested a (partial) and hack-ish support for bun: #59
tests failing for weird reason