[feat] Add README and fully test & improve the code for Nginx plugin for Casdoor

[hsluoyz]

We should implement "Single Sign-On with Casdoor" for Nginx, refer to: https://docs.nginx.com/nginx/deployment-guides/single-sign-on/

Some other resources:

• https://docs.nginx.com/nginx/deployment-guides/single-sign-on/keycloak/
• https://gist.github.com/mjbnz/b402edf819a69e517b0c59710f291da9
• https://github.com/Luzifer/nginx-sso

For how gateway + SSO works, refer to: https://github.com/casdoor/casdoor/issues/905

[casbin-bot]

@seriouszyx @ComradeProgrammer @Resulte

[casbin-bot]

@seriouszyx @ComradeProgrammer @Resulte

[SkipperQ]

I'd like to undertake it.