Add config setting for c8y.enable.log_management

Till c8y-log-plugin is deprecated, one has to explicitly turn on the log
management capability before starting the mapper.

```
$ sudo tedge config set c8y.enable.log_management true
$ tegde-mapper c8y &
$ tedge-log-plugin &
```

Signed-off-by: Didier Wenzek <didier.wenzek@free.fr>

crates/common/tedge_config/src/tedge_config_cli/tedge_config.rs

@@ -385,6 +385,13 @@ define_tedge_config! {
         #[tedge_config(default(value = "te/+/+/+/+/m/+,tedge/alarms/+/+,tedge/alarms/+/+/+,tedge/events/+,tedge/events/+/+,tedge/health/+,tedge/health/+/+"))]
         topics: TemplatesSet,
 
+        enable: {
+            /// Enable log management
+            // TODO turn the default to true, when c8y-log-plugin will be deprecated
+            #[tedge_config(example = "true", default(value = false))]
+            log_management: bool,
+        }
+
     },
 
     #[tedge_config(deprecated_name = "azure")] // for 0.1.0 compatibility

crates/extensions/c8y_mapper_ext/src/config.rs

@@ -82,8 +82,9 @@ impl C8yMapperConfig {
 
         let tedge_http_host = format!("{}:{}", tedge_http_address, tedge_http_port);
 
-        // TODO read these capabilities from disk
-        let capabilities = Capabilities::default();
+        let capabilities = Capabilities {
+            log_management: tedge_config.c8y.enable.log_management,
+        };
 
         let mut topics = Self::default_internal_topic_filter(&config_dir)?;