pytest: add schema support for JSON responses.

This adds our first (basic) schema, and sews support into pyln-testing
so it will load schemas for any method for doc/schemas/{method}.schema.json.

All JSON responses in a test run are checked against the schema (if any).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/schemas/listpays.schema.json

@@ -0,0 +1,72 @@
+{
+    "$schema": "http://json-schema.org/draft-07/schema#",
+    "type": "object",
+    "additionalProperties": false,
+    "properties": {
+	"pays": {
+	    "type": "array",
+	    "items": {
+		"type": "object",
+		"additionalProperties": false,
+		"properties": {
+		    "payment_hash": {
+			"type": "hex",
+			"description": "the hash of the *payment_preimage* which will prove payment",
+			"maxLength": 64,
+			"minLength": 64
+		    },
+		    "status": {
+			"type": "string",
+			"enum": [ "pending", "failed", "complete" ],
+			"description": "status of the payment"
+		    },
+		    "destination": {
+			"type": "pubkey",
+			"description": "the final destination of the payment if known"
+		    },
+		    "amount_msat": {
+			"type": "msat",
+			"description": "the amount the destination received, if known (**status** *complete* or *pending*)"
+		    },
+		    "amount_sent_msat": {
+			"type": "msat",
+			"description": "the amount we actually sent, including fees (**status** *complete* or *pending*)"
+		    },
+		    "created_at": {
+			"type": "u64",
+			"description": "the UNIX timestamp showing when this payment was initiated"
+		    },
+		    "preimage": {
+			"type": "hex",
+			"description": "proof of payment, only if (and always if) **status** is *complete*",
+			"FIXME": "we should enforce the status/payment_preimage relation in the schema!",
+			"maxLength": 64,
+			"minLength": 64
+		    },
+		    "label": {
+			"type": "string",
+			"description": "the label, if given to sendpay"
+		    },
+		    "bolt11": {
+			"type": "string",
+			"description": "the bolt11 string (if pay supplied one)"
+		    },
+		    "bolt12": {
+			"type": "string",
+			"description": "the bolt12 string (if supplied for pay: **experimental-offers** only)."
+		    },
+		    "erroronion": {
+			"type": "hex",
+			"description": "the error onion returned on failure, if any."
+		    },
+		    "number_of_parts": {
+			"type": "u64",
+			"description": "the number of parts for a successful payment (only if more than one, and **status** is *complete*)."
+		    }
+		},
+		"required": [ "payment_hash", "status", "created_at" ]
+	    }
+	}
+    },
+    "required": [ "pays" ]
+}