Part of https://github.com/elastic/kibana/issues/154715

The `.fleet-secrets` system index should just have a value key. 

Kibana system should only have write and delete access NOT read access.

Fleet server will have read access only. 

Example document:

```
// example document from the .fleet-secrets index 
{
	_id: 'abcd1234'
	_source: {
	   value: "abcd12345678",
      }
}
```
 
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authz/store/ReservedRolesStore.java
"logstash_admin",
                new RoleDescriptor(
                    "logstash_admin",
                    new String[] { "manage_logstash_pipelines" },
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices(".logstash*")
                            .privileges("create", "delete", "index", "manage", "read")
                            .allowRestrictedIndices(true)
                            .build() },
                    null,
                    MetadataUtils.DEFAULT_RESERVED_METADATA
                )
            )
            .put(
                "rollup_user",
                new RoleDescriptor("rollup_user", new String[] { "monitor_rollup" }, null, null, MetadataUtils.DEFAULT_RESERVED_METADATA)
            )
            .put(
                "rollup_admin",
                new RoleDescriptor("rollup_admin", new String[] { "manage_rollup" }, null, null, MetadataUtils.DEFAULT_RESERVED_METADATA)
            )
            .put(
                "snapshot_user",
                new RoleDescriptor(
                    "snapshot_user",
                    new String[] { "create_snapshot", GetRepositoriesAction.NAME },
                    new RoleDescriptor.IndicesPrivileges[] {
                        RoleDescriptor.IndicesPrivileges.builder()
                            .indices("*")