RFR: Add support for offline and local builds

[Erik Helin]

Hi all,

this patch adds much improved support for both _offline_ and _local_ builds. We are currently downloading both JDK and Gradle distributions for the most common platforms (GNU/Linux x64, macOS x64 and Windows x64), but this scheme does not scale to multiple other operating systems and/or CPU architectures. I have therefore extended the `images` plugin to support a `local` image which uses the JDK that is currently executing Gradle for producing the jlinked image.

I also added support for fully offline builds for those want to supply both a JDK and Gradle themselves and ensure that the build does not download anything.

Thanks,
Erik

## Testing
- Tested multiple variants of builds, including local, offline and default builds

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Commits:
 - 66a6b56b: Added support for offline and local builds

Changes: https://git.openjdk.java.net/skara/pull/237/files
 Webrev: https://webrevs.openjdk.java.net/skara/237/webrev.00
  Stats: 300 lines in 10 files changed: 207 ins; 22 del; 71 mod
  Patch: https://git.openjdk.java.net/skara/pull/237.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/237/head:pull/237

PR: https://git.openjdk.java.net/skara/pull/237