lightningd: have makesecret take hex or string (just like datastore)

Changelog-Added: JSON-RPC: `makesecret` can take a string argument instead of hex.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-makesecret.7.md

@@ -4,14 +4,15 @@ lightning-makesecret -- Command for deriving pseudorandom key from HSM
 SYNOPSIS
 --------
 
-**makesecret** *hex*
+**makesecret** [*hex*] [*string*]
 
 DESCRIPTION
 -----------
 
 The **makesecret** RPC command derives a secret key from the HSM_secret.
 
-The *hex* can be any hex data.
+One of *hex* or *string* must be specified: *hex* can be any hex data,
+*string* is a UTF-8 string interpreted literally.
 
 RETURN VALUE
 ------------
@@ -32,11 +33,6 @@ AUTHOR
 
 Aditya <<aditya.sharma20111@gmail.com>> is mainly responsible.
 
-SEE ALSO
---------
-
-lightning-getsharedsecret(7)
-
 RESOURCES
 ---------
 

doc/schemas/makesecret.request.json

@@ -2,13 +2,15 @@
   "$schema": "http://json-schema.org/draft-07/schema#",
   "type": "object",
   "additionalProperties": false,
-  "required": [
-    "hex"
-  ],
+  "required": [],
   "properties": {
     "hex": {
       "type": "hex",
       "description": "This will be used for deriving the secret"
+    },
+    "string": {
+      "type": "string",
+      "description": "This will be used for deriving the secret"
     }
   }
 }