Add Custom User Store Listeners section to the docs

[mpmadhavig]

Describe

The Custom User Store Listeners section [1] is missing from IS 7 docs. This issue will track on-boarding of the documentation content of that section.

Action Items

• [ ] Add the wso2-user-store-listener to the sample-is repo
• [ ] Add doc content in IS 6.1 to IS 7.0

[1] https://is.docs.wso2.com/en/6.1.0/references/extend/user-mgt/user-store-listeners/