ipn/ipnlocal/netmapcache: add a package to split and cache network maps (#18497)

This commit is based on part of #17925, reworked as a separate package.

Add a package that can store and load netmap.NetworkMap values in persistent
storage, using a basic columnar representation. This commit includes a default
storage interface based on plain files, but the interface can be implemented
with more structured storage if we want to later.

The tests are set up to require that all the fields of the NetworkMap are
handled, except those explicitly designated as not-cached, and check that a
fully-populated value can round-trip correctly through the cache.  Adding or
removing fields, either in the NetworkMap or in the cached representation, will
trigger either build failures (e.g., for type mismatch) or test failures (e.g.,
for representation changes or missing fields). This isn't quite as nice as
automatically updating the representation, which I also prototyped, but is much
simpler to maintain and less code.

This commit does not yet hook up the cache to the backend, that will be a
subsequent change.

Updates #12639

Change-Id: Icb48639e1d61f2aec59904ecd172c73e05ba7bf9
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

types/netmap/netmap.go

@@ -27,6 +27,8 @@ import (
 // The fields should all be considered read-only. They might
 // alias parts of previous NetworkMap values.
 type NetworkMap struct {
+	Cached bool // whether this NetworkMap was loaded from disk cache (as opposed to live from network)
+
 	SelfNode tailcfg.NodeView
 	AllCaps  set.Set[tailcfg.NodeCapability] // set version of SelfNode.Capabilities + SelfNode.CapMap
 	NodeKey  key.NodePublic