feat(go)!: rework async I/O

- Remove `Completer` abstraction and just always send async values as
  "pending" in Go. We may want to revisit this in the future with a
  better approach (e.g. encoding nested async values concurrently and
  using a `select` to synchronise at encoding position)
- Expose `Close` for stream and future types, although for now the
  callers have to assert for it.

This is not the final solution, just an intermediate state.
More breaking changes are incoming for this functionality

Signed-off-by: Roman Volosatovs <rvolosatovs@riseup.net>

Author: rvolosatovs

crates/wit-bindgen-go/src/interface.rs

@@ -7,9 +7,9 @@ import (
 	"log/slog"
 	"os"
 
+	"github.com/nats-io/nats.go"
 	"wrpc.io/examples/go/hello-client/bindings/wrpc_examples/hello/handler"
 	wrpcnats "wrpc.io/go/nats"
-	"github.com/nats-io/nats.go"
 )
 
 func run() (err error) {

examples/go/hello-client/cmd/hello-client-nats/main.go

@@ -9,9 +9,9 @@ import (
 	"os/signal"
 	"syscall"
 
+	"github.com/nats-io/nats.go"
 	server "wrpc.io/examples/go/hello-server/bindings"
 	wrpcnats "wrpc.io/go/nats"
-	"github.com/nats-io/nats.go"
 )
 
 type Handler struct{}

examples/go/hello-server/cmd/hello-server-nats/main.go

@@ -10,12 +10,12 @@ import (
 	"sync"
 	"syscall"
 
+	"github.com/google/uuid"
+	"github.com/nats-io/nats.go"
 	server "wrpc.io/examples/go/resources-server/bindings"
 	"wrpc.io/examples/go/resources-server/bindings/exports/wrpc_examples/resources/resources"
 	wrpc "wrpc.io/go"
 	wrpcnats "wrpc.io/go/nats"
-	"github.com/google/uuid"
-	"github.com/nats-io/nats.go"
 )
 
 type Foo struct {