Add fips attribute to .fleet-agents docs (#123406)

Co-authored-by: Julien Lind <julien.lind@elastic.co>

Author: michel-laterman

x-pack/plugin/core/template-resources/src/main/resources/fleet-agents.json

@@ -102,6 +102,9 @@
                     "unprivileged": {
                       "type": "boolean"
                     },
+                    "fips": {
+                      "type": "boolean"
+                    },
                     "version": {
                       "type": "text",
                       "fields": {