Make it possible to completely override toolchain support

[melix]

Gradle's toolchain support currently causes a couple of issues:

• it's not possible to select a specific version of GraalVM, for example to use 21.3 if 20.2 is also installed
• it cannot detect GraalVM Enterprise, which completely breaks this plugin

In effect, the plugin makes use of the vendor spec to match GraalVM. This works fine for GraalVM Community Edition, but the string for GraalVM Enterprise is actually "Oracle Corporation", which makes it impossible to differentiate from other Oracle JDKs.

In practice, it's actually the implementation string which makes the difference, but there's no way to ask Gradle for a specific implementation in the current Java toolchain spec.

As a consequence, we should add a way to bypass toolchain detection. Because it works in some cases, it would be a problem to always use the GRAALVM_HOME environment variable to override whatever is declared, so we suggest to add support for a boolean flag on the native image tasks which, when set, would override detection.