cmd/tailscale/cli,ipn/conffile: add declarative config mode for Services (#17435)

This commit adds the subcommands `get-config` and `set-config` to Serve,
which can be used to read the current Tailscale Services configuration
in a standard syntax and provide a configuration to declaratively apply
with that same syntax.

Both commands must be provided with either `--service=svc:service` for
one service, or `--all` for all services. When writing a config,
`--set-config --all` will overwrite all existing Services configuration,
and `--set-config --service=svc:service` will overwrite all
configuration for that particular Service. Incremental changes are not
supported.

Fixes tailscale/corp#30983.

cmd/tailscale/cli: hide serve "get-config"/"set-config" commands for now

tailscale/corp#33152 tracks unhiding them when docs exist.

Signed-off-by: Naman Sood <mail@nsood.in>

tailcfg/tailcfg.go

@@ -17,6 +17,7 @@ import (
 	"net/netip"
 	"reflect"
 	"slices"
+	"strconv"
 	"strings"
 	"time"
 
@@ -1478,6 +1479,15 @@ func (pr PortRange) Contains(port uint16) bool {
 
 var PortRangeAny = PortRange{0, 65535}
 
+func (pr PortRange) String() string {
+	if pr.First == pr.Last {
+		return strconv.FormatUint(uint64(pr.First), 10)
+	} else if pr == PortRangeAny {
+		return "*"
+	}
+	return fmt.Sprintf("%d-%d", pr.First, pr.Last)
+}
+
 // NetPortRange represents a range of ports that's allowed for one or more IPs.
 type NetPortRange struct {
 	_     structs.Incomparable