feat: add sentinel example

[Skyenought]

What type of PR is this?

feat: add sentinel example

What this PR does / why we need it (en: English/zh: Chinese):

zh: 添加 kitex 使用 sentinel-golang 的示例

[li-jin-gou]

Inaccurate title

build: Changes that affect the build system or external dependencies (example scopes: gulp, broccoli, npm)
ci: Changes to our CI configuration files and scripts (example scopes: Travis, Circle, BrowserStack, SauceLabs)
docs: Documentation only changes
feat: A new feature
optimize: A new optimization
fix: A bug fix
perf: A code change that improves performance
refactor: A code change that neither fixes a bug nor adds a feature
style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
test: Adding missing tests or correcting existing tests
chore: Changes to the build process or auxiliary tools and libraries such as documentation generation