[Observability Onboarding] Show already created custom logs integrations in list of available Quickstart flows

[thomheymann]

• When a user goes through the Custom Logs Onboarding flow we create a custom integration for them behind the scenes.
• When a they then try to go through the onboarding flow again to ingest the same log files from a different host that would create an error saying that an integration with that name already exists.
• Instead of forcing users to work around this issue by creating a duplicate integration with a different name we could let them select the ones they already created directly from the landing page.

[elasticmachine]

Pinging @elastic/obs-ux-logs-team (Team:obs-ux-logs)