multi: refactor SignDigestCompact into SignMessageCompact

To make it possible to use a remote lnrpc server as a signer for our
wallet, we need to change our main interface to sign the message instead
of the message's digest. Otherwise we'd need to alter the
lnrpc.SignMessage RPC to accept a digest instead of only the message
which has security implications.

lnrpc/lightning.swagger.json

@@ -6187,6 +6187,10 @@
           "type": "string",
           "format": "byte",
           "description": "The message to be signed. When using REST, this field must be encoded as\nbase64."
+        },
+        "single_hash": {
+          "type": "boolean",
+          "description": "Instead of the default double-SHA256 hashing of the message before signing,\nonly use one round of hashing instead."
         }
       }
     },