Create a ECJ version of BlazeJavaCompilerPlugin

The Bazel javac compiler supports plugins via BlazeJavaCompilePlugin. We need to create a version of this, which allows integration with ECJ.

Specifically the following two methods are offered by BlazeJavaCompilePlugin:

/**
 * Performs analysis actions after the attribute phase of the javac compiler. The attribute phase
 * performs symbol resolution on the parse tree.
 *
 * @param env The attributed parse tree (after symbol resolution)
 */
public void postAttribute(Env<AttrContext> env) {}

/**
 * Performs analysis actions after the flow phase of the javac compiler. The flow phase performs
 * dataflow checks, such as finding unreachable statements.
 *
 * @param env The attributed parse tree (after symbol resolution)
 */
public void postFlow(Env<AttrContext> env) {}

This work also includes patching ECJ to allow contributing such plug-ins programmatically.

Start with the linked BlazeJavaCompilerPlugin.java and compare its postAttribute and postFlow hooks with ECJ's compiler extension points. Then identify where ECJ must be patched to accept plugins programmatically. Done means ECJ exposes equivalent hooks and the plugin can be integrated through the Bazel toolchain.