Allow additional jlink arguments

[commi]

Our app only needs German and English, but a customized JRE always contains the locale data of every language: jdk.localedata is 21.5 MB of the image. jlink --include-locales would drop it, but there is no way to pass that option, because BundleJre builds the jlink command line from fixed arguments.

Measured on our app with JDK 17:

JRE                                                    on disk   zipped
full JRE                                               135 MB    44.9 MB
customizedJre, --compress=0 --include-locales=de,en     92 MB    32.9 MB

This PR adds an additionalJlinkArgs property for Maven and Gradle, appended to the jlink call:

<additionalJlinkArgs>
    <additionalJlinkArg>--include-locales=de,en</additionalJlinkArg>
</additionalJlinkArgs>

Since the arguments come last, a --compress given there also overrides the default --compress=2. That helps installers: an image compressed by jlink can't be compressed again by the zipball or installer. Nothing changes for existing builds.

[fvarrui]

Merged, thanks Till! 🙏

Before merging I tested it with JDK 11, 17, 21 and 25 (Maven, GNU/Linux):

• The arguments reach jlink and are applied: with --include-locales=en,es only the en/es locale data is left.
• A --compress given there overrides the default one, as you said.
• --include-locales fails unless jdk.localedata is in the image, and jdeps usually doesn't detect it, so it has to be added with additionalModules.
• --compress=zip-N only works on JDK 21+; 0/1/2 work everywhere but are deprecated since 21.

I added both notes to the README. The tests also showed that a customized JRE failed on JDK 24+ builds without jmods (e.g. Temurin 25, JEP 493). That's fixed in devel now (jlink links from the run-time image), and the default --compress=2 is now decided by the packaging JDK's version instead of the running one.

It'll be in 2.0.0.