wasi-http: Consider asserting that the `TE` header value can only be `trailers`

I think @lukewagner is planning to propose this upstream for `wasi:http` but with #11002 just landed it would be good to do this sooner than later just to reduce churn across releases.

For context, we removed `TE` from the forbidden headers list partly because "it was the right thing to do" but in practice mostly because [gRPC requires it](https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md).

With [HTTP/2](https://httpwg.org/specs/rfc9113.html#rfc.section.8.2.2) and [HTTP/3](https://httpwg.org/specs/rfc9114.html#rfc.section.4.2) the only valid value for `TE` is `trailers`, so from the perspective of `wasi:http` representing an abstraction over any HTTP version it makes some sense to have this restriction.