JVM: merge manifests of shaded libraries instead of overwriting them when packing benchmarks jar

[fzhinkin]

Currently, JVM's Jar task responsible for generating executable benchmarks jar generates a brand new manifest that includes only a main-class attribute.

Depending on shaded libraries, that might be an issue. For example, if one of the libraries bundled into a resulting jar was a MR-JAR, the resulting manifest will be missing multi-release: true attribute and target-specific code will never be invoked (and that's a problem if you're actually trying to benchmark MR-related functionality ;) ).

It would be nice to merge manifests when generating a jar, like Maven Shade's AppendingTransformer does.

[fzhinkin]

As a workaround, one can update a config of existing Jar task to add required attributes:

afterEvaluate {
    tasks.named("jvmBenchmarkJar").configure {
        this as org.gradle.jvm.tasks.Jar
        manifest {
            attributes("Multi-Release" to true)
        }
    }
}

Besides looking like a hack, the workaround requires to spot the issue in the first place (and that might not be obvious).