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
null,
        new String[] { "*" },
        MetadataUtils.DEFAULT_RESERVED_METADATA,
        Collections.emptyMap(),
        TcpTransport.isUntrustedRemoteClusterEnabled()
            ? new RoleDescriptor.RemoteIndicesPrivileges[] {
                new RoleDescriptor.RemoteIndicesPrivileges(
                    RoleDescriptor.IndicesPrivileges.builder().indices("*").privileges("all").allowRestrictedIndices(false).build(),
                    "*"
                ),
                new RoleDescriptor.RemoteIndicesPrivileges(
                    RoleDescriptor.IndicesPrivileges.builder()
                        .indices("*")
                        .privileges("monitor", "read", "view_index_metadata", "read_cross_cluster")
                        .allowRestrictedIndices(true)
                        .build(),
                    "*"
                ) }
            : null
    );
    private static final Map<String, RoleDescriptor> RESERVED_ROLES = initializeReservedRoles();
    private static RoleDescriptor.RemoteIndicesPrivileges getRemoteIndicesReadPrivileges(String indexPattern) {
        return new RoleDescriptor.RemoteIndicesPrivileges(
            RoleDescriptor.IndicesPrivileges.builder()
                .indices(indexPattern)
                .privileges("read", "read_cross_cluster")
                .allowRestrictedIndices(false)
                .build(),
            "*"
        );