Extend merge_ota with extra options for signapk.jar

Add the same parameters for supporting custom signapk.jar
to merge_ota as already supported by ota_from_targetfiles
introduced in: a28acc69

This makes it possible for an oem to use a custom signapk.jar
for the package signing when using merge_ota.

Bug: 384969259
Test: local
Change-Id: Ia2713f5b0007d6aee34d9546396d744fb5fe6943