version, cmd/tailscale: add version.Meta, tailscale version --json

Add `tailscale version --json` JSON output mode. This will be used
later for a double-opt-in (per node consent like Tailscale SSH +
control config) to let admins do remote upgrades via `tailscale
update` via a c2n call, which would then need to verify the
cmd/tailscale found on disk for running tailscale update corresponds
to the running tailscaled, refusing if anything looks amiss.

Plus JSON output modes are just nice to have, rather than parsing
unstable/fragile/obscure text formats.

Updates #6995
Updates #6907

Change-Id: I7821ab7fbea4612f4b9b7bdc1be1ad1095aca71b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tailcfg/tailcfg_export_test.go

@@ -0,0 +1,7 @@
+// Copyright (c) 2023 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tailcfg
+
+var ExportKeyMarshalText = keyMarshalText

tailcfg/tailcfg_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-package tailcfg
+package tailcfg_test
 
 import (
 	"encoding"
@@ -16,6 +16,7 @@ import (
 	"testing"
 	"time"
 
+	. "tailscale.com/tailcfg"
 	"tailscale.com/tstest"
 	"tailscale.com/types/key"
 	"tailscale.com/util/must"
@@ -660,7 +661,7 @@ func BenchmarkKeyMarshalText(b *testing.B) {
 	b.ReportAllocs()
 	var k [32]byte
 	for i := 0; i < b.N; i++ {
-		sinkBytes = keyMarshalText("prefix", k)
+		sinkBytes = ExportKeyMarshalText("prefix", k)
 	}
 }
 
@@ -670,7 +671,7 @@ func TestAppendKeyAllocs(t *testing.T) {
 	}
 	var k [32]byte
 	err := tstest.MinAllocsPerRun(t, 1, func() {
-		sinkBytes = keyMarshalText("prefix", k)
+		sinkBytes = ExportKeyMarshalText("prefix", k)
 	})
 	if err != nil {
 		t.Fatal(err)