signtool: Add 'sign_archive' command

Signs an archive of GBL images.

Bug: 410495718
Test: ./tools/bazel run @gbl//signtool:gblsigntool -- sign_archive --avbtool_args '--key bootable/libbootloader/gbl/signtool/testkeys/testkey_RSA4096.pem --algorithm SHA256_RSA4096' out/gbl_efi/gbl-img.zip -o signed.zip
Merged-In: I546a6860f70a0621fbbc67204f7567824a479ae0
Change-Id: I546a6860f70a0621fbbc67204f7567824a479ae0