feat(Mailjet Node): Add credential tests and support for sandbox, JSON parameters & variables (#2987)

* Add Variables JSON to Mailjet Batch send

* ⚡ Improvements

* ⚡ Add credential verification

* ⚡ Small improvement

Co-authored-by: Marcin Koziej <marcin@cahoots.pl>

packages/nodes-base/credentials/MailjetEmailApi.credentials.ts

@@ -20,5 +20,12 @@ export class MailjetEmailApi implements ICredentialType {
 			type: 'string',
 			default: '',
 		},
+		{
+			displayName: 'Sandbox Mode',
+			name: 'sandboxMode',
+			type: 'boolean',
+			default: false,
+			description: 'Allow to run the API call in a Sandbox mode, where all validations of the payload will be done without delivering the message',
+		},
 	];
 }