New processor: truncate_fields (elastic#11297)

This PR introduces a new processor `truncate_fields`. To keep raw messages use this processor with `copy_fields`.

### `truncate_fields`

This processor truncates configured fields. Example configuration is below:

```yaml
processors:
- truncate_fields:
    fields:
      - message
    max_bytes: 1024
    fail_on_error: false
    ignore_missing: true
```

### Keep raw events

This preserves the orignal field and truncates it, if it's too long.

```yaml
processors:
- copy_fields:
    fields:
        - from: message
          to: event.original
    fail_on_error: false
    ignore_missing: true
- truncate_fields:
    fields:
      - event.original
    max_bytes: 1024
    fail_on_error: false
    ignore_missing: true
```

journalbeat.reference.yml

@@ -279,6 +279,32 @@ setup.template.settings:
 #          to: message_copied
 #    fail_on_error: true
 #    ignore_missing: false
+#
+# The following example truncates the value of message to 1024 bytes
+#
+#processors:
+#- truncate_fields:
+#    fields:
+#      - message
+#    max_bytes: 1024
+#    fail_on_error: false
+#    ignore_missing: true
+#
+# The following example preserves the raw message under event.original
+#
+#processors:
+#- copy_fields:
+#    fields:
+#        - from: message
+#          to: event.original
+#    fail_on_error: false
+#    ignore_missing: true
+#- truncate_fields:
+#    fields:
+#      - event.original
+#    max_bytes: 1024
+#    fail_on_error: false
+#    ignore_missing: true
 
 #============================= Elastic Cloud ==================================