rpc, p2p/simulations: use github.com/gorilla/websocket (#20289)

* rpc: improve codec abstraction

rpc.ServerCodec is an opaque interface. There was only one way to get a
codec using existing APIs: rpc.NewJSONCodec. This change exports
newCodec (as NewFuncCodec) and NewJSONCodec (as NewCodec). It also makes
all codec methods non-public to avoid showing internals in godoc.

While here, remove codec options in tests because they are not
supported anymore.

* p2p/simulations: use github.com/gorilla/websocket

This package was the last remaining user of golang.org/x/net/websocket.
Migrating to the new library wasn't straightforward because it is no
longer possible to treat WebSocket connections as a net.Conn.

* vendor: delete golang.org/x/net/websocket

* rpc: fix godoc comments and run gofmt

rpc/stdio.go

@@ -33,7 +33,7 @@ func DialStdIO(ctx context.Context) (*Client, error) {
 // DialIO creates a client which uses the given IO channels
 func DialIO(ctx context.Context, in io.Reader, out io.Writer) (*Client, error) {
 	return newClient(ctx, func(_ context.Context) (ServerCodec, error) {
-		return NewJSONCodec(stdioConn{
+		return NewCodec(stdioConn{
 			in:  in,
 			out: out,
 		}), nil