add PKCS11 support (#1153)

* add PKCS11 support

* add pkcs11 build option to the makefile, add a stub pkclient to avoid forcing CGO onto people

* don't print the pkcs11 option on nebula-cert keygen if not compiled in

* remove linux-arm64-pkcs11 from the all target to fix CI

* correctly serialize ec keys

* nebula-cert: support PKCS#11 for sign and ca

* fix gofmt lint

* clean up some logic with regard to closing sessions

* pkclient: handle empty correctly for TPM2

* Update Makefile and Actions

---------

Co-authored-by: Morgan Jones <me@numin.it>
Co-authored-by: John Maguire <contact@johnmaguire.me>

cmd/nebula-cert/p11_stub.go

@@ -0,0 +1,16 @@
+//go:build !cgo || !pkcs11
+
+package main
+
+import (
+	"flag"
+)
+
+func p11Supported() bool {
+	return false
+}
+
+func p11Flag(set *flag.FlagSet) *string {
+	var ret = ""
+	return &ret
+}