Additions to support gRPC

Recently, I've been trying to use a popular .NET library, Grpc.Net, to make outbound gRPC connections using a port of System.Net.Http to wasi-http. I was able to get it to work, but had to hack a few things:

• Patched wasmtime-wasi-http to use HTTP/2 for all outgoing requests
• Patched System.Net.Http to assume incoming responses use HTTP/2
• Patched wasmtime-wasi-http's is_forbidden_header to allow all headers

Ideally, none of those hacks would be necessary, so I'd like to propose the following additions to the wasi-http spec:

• (Optional) Add documentation to clarify that the set of "forbidden" headers may vary among implementations. E.g. browsers might prohibit TE headers, but wasmtime-wasi-http might allow them (and indeed should allow them).
• Add a method on outgoing-request to specify the HTTP version to use. The implementation should return an error if it can't guarantee that version will be used.
• Add a method on incoming-response to retrieve the HTTP version of the response, e.g. as an option<http-version> where http-version is an enum type and none means the version is not known. This allows the client to detect if the server downgraded the connection to a lower version.

See also https://github.com/bytecodealliance/wasmtime/pull/7538 for further discussion.

Start with the wasi-http specification and review the linked Wasmtime PR 7538 plus the referenced runtime-lab PR to understand the existing HTTP/2 and header behavior. Define the specification changes for forbidden headers, outgoing HTTP-version selection, and incoming-response version reporting; done means the proposal is resolved and the specification is updated.