feat: Add Contact::get_or_gen_color. Use it in CFFI and JSON-RPC to avoid gray self-color (#7374)

`Contact::get_color()` returns gray if own keypair doesn't exist yet and we don't want any UIs
displaying it. Keypair generation can't be done in `get_color()` or `get_by_id_optional()` to avoid
breaking Core tests on key import. Also this makes the API clearer, pure getters shouldn't modify
any visible state.

deltachat-ffi/src/lib.rs

@@ -4240,7 +4240,17 @@ pub unsafe extern "C" fn dc_contact_get_color(contact: *mut dc_contact_t) -> u32
         return 0;
     }
     let ffi_contact = &*contact;
-    ffi_contact.contact.get_color()
+    let ctx = &*ffi_contact.context;
+    block_on(async move {
+        ffi_contact
+            .contact
+            // We don't want any UIs displaying gray self-color.
+            .get_or_gen_color(ctx)
+            .await
+            .context("Contact::get_color()")
+            .log_err(ctx)
+            .unwrap_or(0)
+    })
 }
 
 #[no_mangle]