Update the use of 'package:shelf_web_socket's `webSocketHandler` method (#2421)

- add a 2nd argument to the closure passed into
package:shelf_web_socket's `webSocketHandler` method
- widen the dep on package:shelf_web_socket

This will allow us to add more type info to the closure that
`webSocketHandler` expects; it's currently an untyped Function. See also
https://github.com/dart-lang/shelf/issues/457 and
https://github.com/dart-lang/shelf/pull/463.

This forward declares compatibility with `3.0` of
`package:shelf_web_socket`; I _think_ this is necessary - as `dart test`
uses both package:test and package:shelf_web_socket - but happy to hear
otherwise.

---

- [x] I’ve reviewed the contributor guide and applied the relevant
portions to this PR.

<details>
  <summary>Contribution guidelines:</summary><br>

- See our [contributor
guide](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md)
for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before
creating a PR.
- Contributions to our repos should follow the [Dart style
guide](https://dart.dev/guides/language/effective-dart) and use `dart
format`.
- Most changes should add an entry to the changelog and may need to [rev
the pubspec package
version](https://github.com/dart-lang/sdk/blob/main/docs/External-Package-Maintenance.md#making-a-change).
- Changes to packages require [corresponding
tests](https://github.com/dart-lang/.github/blob/main/CONTRIBUTING.md#Testing).

Note that many Dart repos have a weekly cadence for reviewing PRs -
please allow for some latency before initial review feedback.
</details>

---------

Co-authored-by: Jacob MacDonald <jakemac@google.com>