fix: add slf4j dependency

One of the dependencies (zeebe-workflow-engine) requires this dependency. Running the engine without this will result in errors:

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
SLF4J: Failed to load class "org.slf4j.impl.StaticMDCBinder".
SLF4J: Defaulting to no-operation MDCAdapter implementation.
SLF4J: See http://www.slf4j.org/codes.html#no_static_mdc_binder for further details.

Adding this dependency resolves this issue and fixes the logging of the engine.

engine/pom.xml

@@ -46,6 +46,11 @@
       <artifactId>zeebe-client-java</artifactId>
     </dependency>
 
+    <dependency>
+      <groupId>org.slf4j</groupId>
+      <artifactId>slf4j-simple</artifactId>
+    </dependency>
+
     <!-- Test Scope -->
     <dependency>
       <groupId>org.junit.jupiter</groupId>