Add events for organization discovery domains

Feature: Add events for organization discovery domains, such as organization.discovery_domain.created, organization.discovery_domain.deleted etc.

Description: Currently, managing discovery domains between multiple platforms is not possible [at least properly] . Adding these events will allow platforms to independently sync these domains.

Additionally, management API for discovery domains are gated behind organization id, which makes it impossible to uniqueness checks across organizations. For example:

https://{TENANT}.auth0.com/api/v2/organizations/{id}/discovery-domains/name/{discovery_domain}

Requires knowing organization id before making the lookup, an API like so would be much useful to detect non-unique insertion attempts:

https://{TENANT}.auth0.com/api/v2/organizations/discovery-domains/name/{discovery_domain}

So at this point, the only option is to keep discovery domains sync across platforms is to poll management API or some in-house solution.

Use-case: We are managing multiple platforms which allow platform admins to manage their discovery domains.