ZAP Add-ons should typically be implemented in the zap-extensions project.
However there is a way to load them when starting ZAP via the zaproxy project without having to re-build and deploy them each time.
- Build and deploy the extension to zaproxy (or whatever project you are using)
- Edit the "Run Configurations..." * Select the "Classpath" tab * Select the "User Entries" node * Press the "Add Projects..." button * Select the require project, eg zap-extensions-beta
When you run this configuration you should get the latest code changes you've made to the relevant extensions project without having to redeploy the extension.