[Observability Onboarding] Generalize bash-snippet-onboarding

[flash1293]

The existing quickstart flows for logs (system logs and custom logs) work the following way:

To enable similar flows for other integrations, the existing logic in the onboarding plugin needs to be generalized to allow for different flows that follow the same pattern:

• Install integration
• Generate API key
• Provide a downloadable config (either defined locally in the plugin or queried from the fleet plugin)
• Check whether data got indexed

Ideally this logic is a simple allowlist for existing integrations plus minimal additional information:

• Overrides for parts of the config from the fleet plugin (if necessary)
• How to check for incoming data
• Where to redirect the user in case data is available (which dashboard etc.)

Custom logs might stay special here as it's not installing an existing integration but is using a special API to create an integration on demand.

Besides the server side refactoring, the client side components around system and custom log flows should be generalized in the same way.

[flash1293]

Closing as not planned for now