Add support for protoc plugins

Add a proto.plugin property to allow specifying a custom protoc
plugin to generate the code.

Fixes: 70706119
Test: m am StreamingProtoTest
Change-Id: I1ecdd346284b42bbcc8297019d98d2cd564eb94c
diff --git a/java/gen.go b/java/gen.go
index 8362556..500d887 100644
--- a/java/gen.go
+++ b/java/gen.go
@@ -118,7 +118,7 @@
 			javaFile := genLogtags(ctx, srcFile)
 			outSrcFiles = append(outSrcFiles, javaFile)
 		case ".proto":
-			srcJarFile := genProto(ctx, srcFile, flags)
+			srcJarFile := genProto(ctx, srcFile, flags.proto)
 			outSrcFiles = append(outSrcFiles, srcJarFile)
 		case ".sysprop":
 			srcJarFile := genSysprop(ctx, srcFile)