Adds once mechanism for Go 1.21 GOOS=js

This adds an alternative to pooling called once. Specifically, this uses
a module once per request, and in doing so supports guests who cannot
export functions. The implementation adds `await_response` with adapter
glue which the host side implements with Go channels.

As you can see below, this is far less performant than using the pool
model. However, this allows use of Go 1.21 GOOS=wasip1, which does not
support pooling because it cannot export host functions.
```
Benchmark/example_log_once
Benchmark/example_log_once/example_log_once
Benchmark/example_log_once/example_log_once-12                                 	   52648	     23117 ns/op
Benchmark/example_log
Benchmark/example_log/example_log
Benchmark/example_log/example_log-12                                           	 2100068	       550.4 ns/op
```

See golang/go#42372

Signed-off-by: Adrian Cole <adrian@tetrate.io>

api/handler/wasm.go

@@ -300,6 +300,9 @@ const (
 	// TODO: document on http-wasm-abi
 	FuncWriteBody = "write_body"
 
+	// FuncAwaitResponse TODO
+	FuncAwaitResponse = "await_response"
+
 	// FuncGetStatusCode returns the status code produced by FuncNext. This
 	// requires FeatureBufferResponse.
 	//

examples/log_once.wat

@@ -0,0 +1,72 @@
+;; This example module is written in WebAssembly Text Format to show the
+;; how a handler works when the guest compiler doesn't support function
+;; exports, such as GOOS=wasip1 in Go 1.21.
+(module $log_once
+
+  ;; log_enabled returns 1 if the $level is enabled. This value may be cached
+  ;; at request granularity.
+  (import "http_handler" "log_enabled" (func $log_enabled
+    (param $level i32)
+    (result (; 0 or enabled(1) ;) i32)))
+
+  ;; logs a message to the host's logs at the given $level.
+  (import "http_handler" "log" (func $log
+    (param $level i32)
+    (param $buf i32) (param $buf_limit i32)))
+
+(; begin adapter logic
+
+   Below is generic and can convert any normal handler to a single synchronous
+   call, provided $handle_request and $handle_response are not exported. ;)
+
+  ;; import $await_response, which blocks until the response is ready.
+  (import "http_handler" "await_response" (func $await_response
+    (param $ctx_next i64)
+    (result (; is_error ;) i32)))
+
+  ;; define a start function that performs a request-response without exports.
+  ;; note: this logic is generic and can convert any exported $handle_request/
+  ;; $handle_response pair to a synchronous call without exports.
+  (func $start
+    (local $ctx_next i64)
+    (local $is_error i32)
+    (local $ctx i32)
+
+    ;; ctxNext := handleRequest()
+    (local.set $ctx_next (call $handle_request))
+
+    ;; isError := awaitResponse(ctxNext())
+    (local.set $is_error (call $await_response (local.get $ctx_next)))
+
+    ;; expected_count = uint32(result >> 32)
+    (local.set $ctx
+      (i32.wrap_i64 (i64.shr_u (local.get $ctx_next) (i64.const 32))))
+
+    (call $handle_response (local.get $ctx) (local.get $is_error))
+  )
+
+(; end adapter logic ;)
+
+  (memory (export "memory") 1 1 (; 1 page==64KB ;))
+  (global $message i32 (i32.const 0))
+  (data (i32.const 0) "hello world")
+  (global $message_len i32 (i32.const 11))
+
+  (func $handle_request (result (; ctx_next ;) i64)
+    ;; We expect debug logging to be disabled. Panic otherwise!
+    (if (i32.eq
+          (call $log_enabled (i32.const -1)) ;; log_level_debug
+          (i32.const 1)) ;; true
+        (then unreachable))
+
+    (call $log
+      (i32.const 0) ;; log_level_info
+      (global.get $message)
+      (global.get $message_len))
+
+    ;; uint32(ctx_next) == 1 means proceed to the next handler on the host.
+    (return (i64.const 1)))
+
+  ;; handle_response is no-op as this is a request-only handler.
+  (func $handle_response (param $reqCtx i32) (param $is_error i32))
+)