Header values no longer have to be ASCII or UTF-8
<!-- Please describe your changes on the following line: -->
This passes some failed tests related to header validity when handling ByteStrings outside the printable ASCII range. A few failures remain because the HeaderValue class is stricter than WHATWG/WPT, disallowing various control-code bytes that the spec and tests expect to be allowed.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix some of the test cases described in #24903
<!-- Either: -->
- [X] There are tests for these changes OR
<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
Continue http cache work
<!-- Please describe your changes on the following line: -->
Work in progress, and not quite worth a review yet. (Continuation of https://github.com/servo/servo/pull/4117)
TODO
- [ ] cache metadata (find some subset of`net_traits::Metadata` that can be shared across threads, it seems the problem is mainly stuff inside `hyper::header` in the `headers` field)
- [ ] determine which other fields of a `Response` need to be cached, so a full and valid one can be returned upon a cache hit.
- [ ] determine how to best share the cache across fetch threads (inside HttpState like I tried now?)
- [ ] Spend more time reading the spec and make sure the cache follows it where it matters.
- [ ] Make the current wpt tests pass.
- [ ] More...
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix#12972 (github issue number if applicable).
<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____
<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->
<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->
<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18676)
<!-- Reviewable:end -->
