[http] deprecate `request-options` `first-byte-timeout`

I think we should deprecate first-byte-timeout:

• For many HTTP clients (including fetch), the first (non-error) event observable after dispatching a request is the end of response headers (e.g. \r\n\r\n for HTTP/1.1). The first-byte-timeout instead wants to wait for one more byte: "the first byte of the Response body". I'm not familiar with any reason to treat that byte as special.

• End of response headers already has an event: the async return of response. If someone wants a timeout there they can use a regular timer that races the response.

• The timeout between end of response headers and the first byte of the response body can instead be covered by between-bytes-timeout. This would technically be a breaking change but I believe wasmtime-wasi-http and jco already behave this way, so I suggest that we just document that both the original spec behavior and the current implementations' behaviors are acceptable.

Start with proposals/http/wit/types.wit at the request-options definitions, then compare the referenced behavior in crates/wasi-http/src/default_send_request.rs and packages/preview2-shim/src/browser/http.ts. Confirm the intended compatibility wording and update the specification so the deprecation and acceptable timeout behavior are documented consistently.