添加spring-boot-starter模块，简化springboot项目中的配置

[loong95]

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Build-related changes
• [ ] Other, please describe:

The description of the PR:

1. 添加xxl-job-spring-boot-starter模块，简化springboot项目中引入xxl-job-core的配置
2. 将xxl-job-executor-sample/xxl-job-executor-sample-springboot示例修改为引用starter

Other information: