Make environment variables set in the pom.xml of the native-maven-plugin be visible in the image builder

[jormundur00]

In this PR, we make environment variables set in the pom.xml <environmentVariables> tag visible to the native image static analysis by providing them to the native-image builder using the -E<variable>=<value> flag (consistent with how systemProperties are currently handled).

Also in this PR, to avoid documentation mismatches and breaking older builds, we add aliases for both environmentVariables and systemPropertyVariables properties.

Current documentation and samples frequently use these names even though the plugin only recognized the field names environment and systemProperties. By using aliases, we ensure:

• Consistency: The naming now matches the maven-surefire-plugin and maven-failsafe-plugin conventions.
• Backwards Compatibility: Existing builds using the <environment> and <systemProperties> tags will continue to work without modification.

Alternatively we could just rename the fields to environmentVariables and systemPropertyVariables (and avoid aliases), but that could potentially break older builds that used the old tag names.

Fixes: #818

[vjovanov]

Is this even documented? Why not just -D?

[jormundur00]

Currently, the documentation states that the <systemPropertyVariables> tag should be used to set system properties from the pom.xml. Internally, this tag gets mapped as if the user has been using -D from the command line:

if (systemProperties != null) {
    for (Map.Entry<String, String> entry : systemProperties.entrySet()) {
        cliArgs.add("-D" + entry.getKey() + "=" + entry.getValue());
    }
}

[vjovanov]

We need this for bundles?

[jormundur00]

This tag represents a way to set env variables through the pom.xml (like it's done in the surefire/failsafe plugins), without having to provide them in the CLI or through other <buildArgs> -E<..>=<..> pom tags.

The alias was added to stay consistent with other plugins that use this feature (and our documentation) while not breaking older builds that used the <environment> tag.