Add OAuth2 plugin to ShenYu

[tzzz z]

It is planned to add a plug-in for OAuth2 in shenyu. Currently, the tools
that can be integrated are WebFlux Security and this is my design flow
chart: https://gitmind.cn/app/flowchart/8a41654926. I want to discuss the
specific integration plan in the community. If there are other better
suggestions, I hope you can put forward them. This feature suggestion also
see: https://github.com/dromara/shenyu/issues/1401.