add support for JDK toolchains

[hboutemy]

New feature, improvement proposal

Maven Archiver defines META-INF/MANIFEST.MF entries:

• Build-Jdk-Spec
• Build-Jdk

they are taken from Maven runtime: when a JDK toochain is used, we need clarity between JDK/JRE running Maven (the build tool) vs JDK for compiling (running the compiler)

Build-Jdkand Build-Jdk-Spec in spirit are more about he compiler than Maven runtime

Build-Tool-Jdk-Spec (or Jre?) should give the Java version used by Maven build tool runtime, at least when toolchains is used, perhaps also when toolchains are not used, just to start giving good habits of not always considering they are the same
And perhaps do the equivalent for Build-Tool-Jdk

Existing solution in Maven Jar Plugin

this has been partially done in Maven Jar Plugin 3.4.0 MJAR-62:

• adding Build-Tool-Jdk-Spec with runtime value = [MJAR-62] Set Build-Jdk according to used toolchain maven-jar-plugin#73,
• overriding Build-Jdk-Spec with the value from the toolchain
• disabling default manifest entries (which disables Java-Version added in add Java-Version entry to default MANIFEST.MF #298)
• ignoring Build-Jdk: in case of toolchains usage, Build-Jdk gives full java version from Maven runtime, while Build-Jdk-Spec gives spec of toolchain

this algorithm would be better implemented in Maven Archiver, with the plugin just configuring the toolchain to Maven Archiver: then Maven Archiver can use the toolchain

this would probably also be a good first step to injecting later the toolchain to Plexus Module Archiver that seems to be necessary given failure described in apache/maven-jar-plugin#439

[hboutemy]

notice: we finally NEED this approach built in maven-archiver, as for example Maven Archiver added Java-Version entry in 3.6.5, that is not kept by Maven Jar Plugin when it disables Maven Archiver defaults and instead does it its own way