dillon1000/react

[Flight] Allow passing DEV only startTime as an option (#34912)

When you use the `createFromFetch` API we assume that the start time of
the request is the same time as when you call `createFromFetch` but in
principle you could use it with a Promise that starts earlier and just
happens to resolve to a `Response`.

When you use `createFromReadableStream` that is almost definitely the
case. E.g. you might have started it way earlier and you don't call
`createFromReadableStream` until you get the headers back (the fetch
promise resolves).

This adds an option to pass in the start time for debug purposes if you
started the request before starting to parse it.