Add network device driver model, netdev

This PR adds a network device driver model called netdev. There will be a companion PR for TinyGo drivers to update the netdev drivers and network examples. This PR covers the core "net" package.

An RFC for the work is here: #tinygo-org/drivers#487. Some things have changed from the RFC, but nothing major.

The "net" package is a partial port of Go's "net" package, version 1.19.3. The src/net/README file has details on what is modified from Go's "net" package.

Most "net" features are working as they would in normal Go. TCP/UDP/TLS protocol support is there. As well as HTTP client and server support. Standard Go network packages such as golang.org/x/net/websockets and Paho MQTT client work as-is. Other packages are likely to work as-is.

Testing results are here (https://docs.google.com/spreadsheets/d/e/2PACX-1vT0cCjBvwXf9HJf6aJV2Sw198F2ief02gmbMV0sQocKT4y4RpfKv3dh6Jyew8lQW64FouZ8GwA2yjxI/pubhtml?gid=1013173032&single=true).

http/method.go

@@ -0,0 +1,22 @@
+// TINYGO: The following is copied from Go 1.19.3 official implementation.
+
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package http
+
+// Common HTTP methods.
+//
+// Unless otherwise noted, these are defined in RFC 7231 section 4.3.
+const (
+	MethodGet     = "GET"
+	MethodHead    = "HEAD"
+	MethodPost    = "POST"
+	MethodPut     = "PUT"
+	MethodPatch   = "PATCH" // RFC 5789
+	MethodDelete  = "DELETE"
+	MethodConnect = "CONNECT"
+	MethodOptions = "OPTIONS"
+	MethodTrace   = "TRACE"
+)