Add sentinel servlet adapter demo

[marmot-z]

Describe what this PR does / why we need it

Add sentinel servlet adapter demo.

Does this pull request fix one issue?

Describe how you did it

Describe how to verify it

1. Download sentinel project
2. Build sentinel-demo-servlet module (mvn clean package)
3. Copy sentinel-demo-servlet war file to tomcat webapps floder
4. Start tomcat and verify successful registe to sentinel dashboard

Special notes for reviews