A custom APEX signing tool with --signing_args

apex_utils.SignApex() has `signing_args` argument which is supposed to
be passed to avbtool to set `--signing_helper` or
`--signing_helper_with_files`.

A custom APEX signing tool which relies on avbtool's signing should pass
the same arguments when signing_args is set.

This change passes signing_args verbatim to the custom APEX signing tool
via --signing_args argument.

Bug: 216971834
Test: tbd
Change-Id: I8aaec9440c4536e11e1e4a1dd4ae5f4fe32d4f23