Support custom payload signer in merge_ota

Add options to handle custom payload signer as it is required in
merge_ota.py as well if the original OTA packages are signed by
the signer.

If input is only one OTA, clone apex_info.pb to the target.

Use common.ZipWriteStr instead of zipfile.writestr, this ensures
that the same permission for the entries as done by
ota_from_target_files.

Bug: 282189563
Test: manual. pass single OTA to merge_ota, with same signing
    parameters as originally used. Confirm that output zip is
    binary identical to input.
Change-Id: I3b926b8cd69bc74dff6ccf8b5ccc72bedffcac6f