The approach for ids for Flight is different from Fizz/Client where there is a need for determinancy. Flight rendered elements will not be rendered on the client and as such the ids generated in a request only need to be unique. However since FLight does support refetching subtrees it is possible a client will need to patch up a part of the tree rather than replacing the entire thing so it is not safe to use a simple incrementing counter. To solve for this we allow the caller to specify a prefix. On an initial fetch it is likely this will be empty but on refetches or subtrees we expect to have a client useId provide the prefix since it will guaranteed be unique for that subtree and thus for the entire tree. It is also possible that we will automatically provide prefixes based on a client/Fizz useId on refetches

in addition to the core change I also modified the structure of options for renderToReadableStream where onError, context, and the new identifierPrefix are properties of an Options object argument to avoid the clumsiness of a growing list of optional function arguments.