feature/featuretags, all: add ts_omit_acme to disable TLS cert support

I'd started to do this in the earlier ts_omit_server PR but
decided to split it into this separate PR.

Updates #17128

Change-Id: Ief8823a78d1f7bbb79e64a5cab30a7d0a5d6ff4b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/buildfeatures/feature_acme_disabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build ts_omit_acme
+
+package buildfeatures
+
+// HasACME is whether the binary was built with support for modular feature "ACME TLS certificate management".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_acme" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasACME = false

feature/buildfeatures/feature_acme_enabled.go

@@ -0,0 +1,13 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code generated by gen.go; DO NOT EDIT.
+
+//go:build !ts_omit_acme
+
+package buildfeatures
+
+// HasACME is whether the binary was built with support for modular feature "ACME TLS certificate management".
+// Specifically, it's whether the binary was NOT built with the "ts_omit_acme" build tag.
+// It's a const so it can be used for dead code elimination.
+const HasACME = true

feature/featuretags/featuretags.go

@@ -42,6 +42,7 @@ type FeatureMeta struct {
 // Features are the known Tailscale features that can be selectively included or
 // excluded via build tags, and a description of each.
 var Features = map[FeatureTag]FeatureMeta{
+	"acme":             {"ACME", "ACME TLS certificate management"},
 	"aws":              {"AWS", "AWS integration"},
 	"bird":             {"Bird", "Bird BGP integration"},
 	"capture":          {"Capture", "Packet capture"},