Support Buf Registry Login

We are using this plugin and have wired up our own janky way to trigger login when users run a gradle build on their laptops and don't have the BUF_TOKEN set: ```kotlin tasks.register<Exec>("registryAuthCheck") { commandLine("bash", "-lc", "buf registry whoami || buf registry login") } ``` This still requires the user to have the buf CLI installed, despite the gradle plugin already pulling one down in the build directory. It would be great if the plugin provided an optional task we could wire into our builds that would do this.