Add WebTransport support in addition to gRPC-web for Web support

[uldall]

It was announced in Interop 2026 that WebTransport will be supported by all major browsers:
https://webkit.org/blog/17818/announcing-interop-2026/#webtransport

Safari was the only major browser that didn't already support it, but it seems it is now included in 26.4 TP:
https://caniuse.com/webtransport

Adding support for WebTransport would enable this package to allow Flutter Web applications to call HTTP3-enabled gRPC servers without the need for a proxy.

The WebTransport implementation should not be seen as a direct replacement for the gRPC-web implementation as the WebTransport implementation puts other requirements on the server side. Essentially that the server supports HTTP/3 instead of the Envoy proxy that gRPC-web requires.

I would be willing to upstream such an implementation, but it would be beneficial to know that the project is interested in accepting this proposal before undertaking the work.

It would also be interesting to have a discussion on how best to structure the code to support these two different web "backends". I imagine that users would need to decide between using the regular gRPC protocol (With only HTTP/3 support) or the gRPC-web protocol.

[mraleph]

We have fairly limited review capacity. If the change is small enough, well structured, well tested etc - then I think we can review and merge it.