Add initial implementation of OpenSergo Java client

[sczyh30]

Add initial implementation of OpenSergo Java client.

• Add fundamental client subscribe and config cache mechanism
• Add fundamental implementation of OpenSergo gRPC client

[sczyh30]

An example of how data-plane integrates with OpenSergo Java SDK:

OpenSergoClient client = new OpenSergoClient(host, port);
client.start();

// Push-model
client.subscribeConfig(new SubscribeKey(namespace, appName, configKind),
    new OpenSergoConfigSubscriber() {
        @Override
        public boolean onConfigUpdate(SubscribeKey subscribeKey, Object dataList) {
            // Handle received config here
            System.out.println("key: " + subscribeKey + ", data: " + dataList);
            return true;
        }
    });

[jnan806]

reserved tag conflict with docs https://github.com/opensergo/opensergo-specification/blob/main/specification/zh-Hans/README.md
maybe the docs is needed to change the tag range

[jnan806]

是否应该将 *.proto 文件抽取到 opensergo/opensergo-proto 项目

[sczyh30]

是否应该将 *.proto 文件抽取到 opensergo/opensergo-proto 项目

对应的 proto 会提交到 opensergo-proto 项目里面；不过如何在 SDK 项目中比较方便地引用 proto，社区可以给一些更好的建议

[jnan806]

对应的 proto 会提交到 opensergo-proto 项目里面；不过如何在 SDK 项目中比较方便地引用 proto，社区可以给一些更好的建议

refer to opensergo/opensergo-proto#3

[jnan806]

@sczyh30

PTAL！THX.

针对 issue opensergo/opensergo-proto#3

已经将 grpc协议的java和go两中SDK的依赖实现提交在 PR 中，麻烦review并讨论下是否合理。

opensergo/opensergo-proto#4

[sczyh30]

Hi, I've updated the latest proto and polished SDK code (a lot of bugs has been fixed...).

cc @opensergo/opensergo-maintainer

[jnan806]

ok, I've synchronized my fork branch https://github.com/jnan806/opensergo-java-sdk-review-sczyh30/tree/stubcode-based-sczyh30
Please take a review