net/netip: add new IP address package

Co-authored-by: Alex Willmer <alex@moreati.org.uk> (GitHub @moreati)
Co-authored-by: Alexander Yastrebov <yastrebov.alex@gmail.com>
Co-authored-by: David Anderson <dave@natulte.net> (Tailscale CLA)
Co-authored-by: David Crawshaw <crawshaw@tailscale.com> (Tailscale CLA)
Co-authored-by: Dmytro Shynkevych <dmytro@tailscale.com> (Tailscale CLA)
Co-authored-by: Elias Naur <mail@eliasnaur.com>
Co-authored-by: Joe Tsai <joetsai@digital-static.net> (Tailscale CLA)
Co-authored-by: Jonathan Yu <jawnsy@cpan.org> (GitHub @jawnsy)
Co-authored-by: Josh Bleecher Snyder <josharian@gmail.com> (Tailscale CLA)
Co-authored-by: Maisem Ali <maisem@tailscale.com> (Tailscale CLA)
Co-authored-by: Manuel Mendez (Go AUTHORS <mmendez534@...)>
Co-authored-by: Matt Layher <mdlayher@gmail.com>
Co-authored-by: Noah Treuhaft <noah.treuhaft@gmail.com> (GitHub @nwt)
Co-authored-by: Stefan Majer <stefan.majer@gmail.com>
Co-authored-by: Terin Stock <terinjokes@gmail.com> (Cloudflare CLA)
Co-authored-by: Tobias Klauser <tklauser@distanz.ch>

Fixes #46518

Change-Id: I0041f9e1115d61fa6e95fcf32b01d9faee708712
Reviewed-on: https://go-review.googlesource.com/c/go/+/339309


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>