generate etags for binary responses

[Rich-Harris]

#1136. seeing some test failures that I can't explain so I'm not going to merge this, instead I'm going to reimplement it more carefully. Just leaving this here for reference.

it involves a few changes:

• separating ServerResponse from the new EndpointOutput type (both types are public)
• ServerResponse body can be string or Uint8Array (the latter only/always when content-type is application/octet-stream)
• EndpointOutput body can also be a JSONValue
• handle response must be ServerResponse (i.e. body must be string or Uint8Array here, not JSONValue)
• rawBody can be string or Uint8Array (less sure about this bit)

Aside: brilliantly, in Node, when you call res.end(uint8Array) you get the following error:

The "chunk" argument must be of type string or an instance of Buffer or Uint8Array. Received an instance of Uint8Array

Slow clap. It turns out you can do res.write(uint8Array), res.end() though.

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[Rich-Harris]

closing in favour of #1382