lnrpc: lets encrypt

This commit enables lnd to request and renew a Let's Encrypt
certificate. This certificate is used both for the grpc as well as the
rest listeners. It allows clients to connect without having a copy of
the (public) server certificate.

Co-authored-by: Vegard Engen <vegard@engen.priv.no>

sample-lnd.conf

@@ -50,6 +50,20 @@
 ; or want to expose the node at a domain.
 ; externalhosts=my-node-domain.com
 
+; Sets the directory to store Let's Encrypt certificates within
+; letsencryptdir=~/.lnd/letsencrypt
+
+; Sets the port on which lnd will listen for Let's Encrypt challenges. Let's 
+; Encrypt will always try to contact on port 80. Often non-root processes are
+; not allowed to bind to ports lower than 1024. This configuration option allows
+; a different port to be used, but must be used in combination with port
+; forwarding from port 80.
+; letsencryptport=8080
+
+; Request a Let's Encrypt certificate for this domain. Note that the certicate
+; is only requested and stored when the first rpc connection comes in.
+; letsencryptdomain=example.com
+
 ; Disable macaroon authentication. Macaroons are used are bearer credentials to
 ; authenticate all RPC access. If one wishes to opt out of macaroons, uncomment
 ; the line below.