Add release automation with GitHub Actions

Releasing this SDK requires dealing with Maven and GPG, both of which can be finicky and difficult. We should have a manual workflow that we can run from the repo's GitHub Actions that will kick off the release for us. I assume we'll still have to do the final step on oss.sonatype.com manually, but that could be a good thing because we'll be able to verify that it worked correctly.

Steps for release are outlined in RELEASING.md

首先阅读 RELEASING.md，确定 Maven 和 GPG 的发布步骤，以及 oss.sonatype.com 仍需手动操作的环节。然后检查仓库的 GitHub Actions 配置，并定义一个手动触发的 workflow 来执行文档中说明的发布步骤；完成的标准是可以从 GitHub Actions 运行该 workflow，并且只剩下预期的最后一个 Sonatype 步骤。

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