RFR: 8286850: [macos] Add support for signing user provided app image [v2]

[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.

Alexander Matveev has updated the pull request incrementally with one additional commit since the last revision:

  8286850: [macos] Add support for signing user provided app image [v2]

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8987/files
  - new: https://git.openjdk.java.net/jdk/pull/8987/files/384738c1..65df10e9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8987&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8987&range=00-01

  Stats: 37 lines in 3 files changed: 3 ins; 22 del; 12 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