[Metrics UI] Add alerts to the inventory screen

[phillipb]

Add the ability to create alerts for inventory on the inventory screen for metrics.

Tasks:

• Create a new alert type for inventory metric alerts.
• Create a new alert flyout for the inventory screen.
• Create an alert executor that reuses much of the stuff we already do for snapshot api.
• Show threshold measurement type in the expression row.
• Pre-fill flyout with data from the inventory view
• Add alert flyout button to the context menu
• Make sure edit works properly form management screen
• Verify time range works properly

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)