Webhook alert token and new user alerts

[fnxpt]

Description

This PR adds two new notification types (user_created and user_deleted) and the possibility of setting a token on the webhook alert

Addressed Issue

Closes #3255
Closes #3256

Checklist

• I have read and understand the contributing guidelines
• This PR implements an enhancement, and I have provided tests to verify that it works as intended

[fnxpt]

@nscuro couldn't find where is the documentation for these alerts, can you point me to the right place

[valentijnscholten]

@nscuro couldn't find where is the documentation for these alerts, can you point me to the right place

probably here: https://docs.dependencytrack.org/integrations/notifications/#outbound-webhooks
source: https://github.com/DependencyTrack/dependency-track/blob/master/docs/_docs/integrations/notifications.md

[nscuro]

One major caveat here is that users created via auto-provisioning are not captured. As this is happening in Alpine, there's no easy way for DT to detect it:

• Users created for OIDC: https://github.com/stevespringett/Alpine/blob/5727c590c59881d6e46630839e2a8a9dcadcbf4f/alpine-server/src/main/java/alpine/server/auth/OidcAuthenticationService.java#L228-L241
• Users created for LDAP: https://github.com/stevespringett/Alpine/blob/5727c590c59881d6e46630839e2a8a9dcadcbf4f/alpine-server/src/main/java/alpine/server/auth/LdapAuthenticationService.java#L117-L121