Initial java_toolchain Java 9 readiness

Add a generic java_toolchain.javacopts attribute, and make some attributes that
aren't needed for Java 9 optional (source_version, target_version, bootclasspath,
extclasspath). In hindsight there aren't any real advantages to baking details
of javac flags into java_toolchain except for the ones that deal with search paths
(classpath etc.). Using a more general javacopts attribute will avoid churn in the
future as the javac API changes.

This is pre-work for supporting e.g. --release.

PiperOrigin-RevId: 179590640

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