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).
