[whatwg/fetch] Tighten network partition key (at least in "webtransport" mode) (Issue #1926)

jan-ivar created an issue (whatwg/fetch#1926)

### What is the issue with the Fetch Standard?

The [network partition key](https://fetch.spec.whatwg.org/#determine-the-network-partition-key) step says `secondKey` can _"be null or an [implementation-defined](https://infra.spec.whatwg.org/#implementation-defined) value"._

This seems like a historical accommodation, with all major engines partitioning `fetch()` more strictly today.

But for new technology, this seems to leave the door ajar for no reason.

For WebTransport, whose constructor builds a fetch request in `"webtransport"` mode, a `null` secondKey lets a compliant UA [pool](https://fetch.spec.whatwg.org/#concept-connection-obtain) QUIC sessions across cross-origin iframes on the same top-level page. Seems problematic.

No engine ships WT pooling yet, so tightening is free.

Possible avenues:
- A: prescribe a non-null `secondKey` for everyone (e.g the request's client's origin or its site)
  - https://github.com/whatwg/fetch/issues/1035
- B: when request's mode is `"webtransport"`, prescribe A
- C: when request's mode is `"webtransport"` and secondKey doesn't satisfy A, set _new_ to `"yes-and-dedicated"`

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