Implement Streaming Static/Proxy Rendering (#4329)
### Description
Implements streaming rendering for static/proxy renders (I don't know
why they're called static or proxy…) using an initial `Headers` message,
N `BodyChunk` messages, and a final `BodyEnd` message.
Updating proxy rendering was very easy. It already implemented separate
messages for header and body, but it buffered the full body before
sending the full contents. But static rendering was a little more
difficult, it has multiple paths that are immediately ready with a full
body that I didn't want to break apart. So I kept the `Response` message
for a full response that transforms into a
`ContentSourceContent::Static`, and implemented a new enum type for
streaming responses into a `ContentSourceContent::HttpProxy`.
These is some weirdness with different data types on the `headers` field
of a `ContentSourceContent::Static`'s `StaticContent` and a
`ContentSourceContent::HttpProxy`'s `ProxyResultVc`. One is a
`HeaderListVc` and the other is a `Vec<(String, String)>` (and really
should be a `HeadersList`), but I'm leaving this for a follow up.
### Testing Instructions
Paired Next.js PR: https://github.com/vercel/next.js/pull/47476
Re: WEB-27
