RFR: 8286850: [macos] Add support for signing user provided app image

[Alexander Matveev]

- Added support for signing predefined application image.
 - Following command can be used to sign predefined application images: jpackage --type app-image --app-image Test.app --mac-sign [additional signing options]
 - Main class and if --mac-app-store was specified will be saved in .jpackage.xml. Both values are required for signing. Main class is to generate default identity and --mac-app-store to do correct signing for App Store.
 - Signing is done exactly same as when generating app bundle. Unsigned, signed or partially signed app images are supported. App image will be completely unsigned before signing.

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Commit messages:
 - Merge remote-tracking branch 'upstream/master' into JDK-8286850
 - 8286850: [macos] Add support for signing user provided app image

Changes: https://git.openjdk.java.net/jdk/pull/8987/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8987&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286850
  Stats: 960 lines in 27 files changed: 811 ins; 22 del; 127 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8987.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8987/head:pull/8987

PR: https://git.openjdk.java.net/jdk/pull/8987