[Log Explorer] Convert the log explorer profile into a standalone app

[weltenwort]

📓 Summary

Instead of registering a customization profile with Discover, we want to register a separate app with Discover's UX but the logging- and observability-specific applications.

• 🔗 relates to: [Logs+] Implement Logs Dataset selector #159907
• 🛑 blocked by: [Security Solution][Feat] Integrate Discover Tab in timeline #160036

✔️ Acceptance criteria

• The plugin x-pack/plugins/discover_log_explorer is now called log_explorer.
• The log_explorer plugin doesn't register a discover profile anymore.
• The log_explorer plugin is not tied to any observability concepts and doesn't depend on any observability plugin.
• The log_explorer plugin exports a LogExplorer component, which...
  • renders the discover main app component that was extracted by the security team
  • uses the customizations that were previously defined
• There is a new plugin x-pack/plugins/observability_log_explorer, which...
  • registers a new app with a deep link.
  • owns the deepLinkVisible configuration flag to control its exposure in the navigation.
  • is represented in the serverless observability navigation using that deep link.
  • renders the LogExplorer component from the log_explorer plugin inside of the observability page template.

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)