tailcfg: replace int with DERPRegionID for additional type safety (#20646)

Historically, when DERP regions were switched away from strings to
numeric identifiers in PR #14641, tailcfg.Node.HomeDERP was declared
as an int instead of its own type.

This PR declares a new tailcfg.DERPRegionID type, represented by an
int64, and converts the following fields to use this type:

- netcheck.Report.PreferredDERP
- netcheck.Report.RegionLatency
- netcheck.Report.RegionV4Latency
- netcheck.Report.RegionV6Latency
- tailcfg.DERPHomeParams.RegionScore
- tailcfg.DERPMap.Regions
- tailcfg.DERPNode.RegionID
- tailcfg.DERPRegion.RegionID
- tailcfg.NetInfo.PreferredDERP
- tailcfg.Node.HomeDERP
- tailcfg.PeerChange.DERPRegion
- tailcfg.PingResponse.DERPRegionID

Note that the original field was an int, while the new field is backed
by an int64. This change makes DERPRegionID the same size on both
32-bit and 64-bit architectures.

Fixes: #20165

Change-Id: Ic6f795a6d791dd16f756f246d5a02085443e212f

Signed-off-by: Simon Law <sfllaw@tailscale.com>