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

[lann]

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.

With HTTP/2 and HTTP/3 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.