Gradle option to use model file instead of creating/modifying one

Currently, every time someone builds a Gradle project, the plugin will look for the entities and recreate the model JSON file if needed.

My suggestion is to add an option to only read the model file without checking and changing it.

The reason for such an option is to allow sharing the same model file and database in multiple languages.

To be more specific, I have a Flutter app that uses objectbox-dart, that app also has Android widgets written in Kotlin, so the idea is that I want to load an objectbox database in read-only mode inside my widget to get information previously added using objectbox-dart in Flutter.

I'm able to do that if I add this to my build.gradle file:

        javaCompileOptions {
            annotationProcessorOptions {
                arguments.put("objectbox.modelPath", "$rootDir/../lib/objectbox-model.json".toString())
            }
        }

And that works, the issue is that from the Flutter side, I have multiple entities, but from the Kotlin side, I'm only interested in one, so if I build my grade project, my JSON model will be replaced with only the entity shared between both languages, to make the Flutter app work again, I need to always forcefully regenerate the model so it overrides what the gradle plugin changed. Hence why I want to prevent gradle build from changing that file.

先从 Gradle 插件对注解处理器选项 objectbox.modelPath 的处理，以及 issue 中所示的 build.gradle 配置开始。跟踪模型 JSON 在哪里被检查和重写，然后定义一个在不修改现有共享模型的情况下读取它的选项；完成标准是 Kotlin 能够基于 Flutter 模型构建，同时保留所有 Flutter 实体。