feat(jsonrpc): generate OpenRPC definitions

When running `cargo test` in the deltachat-jsonrpc folder,
a new file `openrpc/openrpc.json` will be created with an
[OpenRPC](https://spec.open-rpc.org/) definition.

It can be copy-pasted into the
[OpenRPC playground](https://playground.open-rpc.org/)
and used to generate clients in other languages.

deltachat-jsonrpc/src/api/mod.rs

@@ -144,7 +144,11 @@ impl CommandApi {
     }
 }
 
-#[rpc(all_positional, ts_outdir = "typescript/generated")]
+#[rpc(
+    all_positional,
+    ts_outdir = "typescript/generated",
+    openrpc_outdir = "openrpc"
+)]
 impl CommandApi {
     /// Test function.
     async fn sleep(&self, delay: f64) {