[Security Solution] Url state parameter for external alerts checkbox

[semd]

Describe the feature:

The new external alerts checkbox, that allows filtering the events table and histogram to show external alerts only, does not currently survive URL sharing because it is not persisted in the Url.
This is a functionality that we had in the previous External Alerts tab, since all tabs in Security are persisted in the URL as part of the path.

Describe a specific use case for the feature:

Make the external alerts check persistent in the URL via UrlState, so when users share a link while having the external alerts checkbox enabled, the link will open the page with the external alerts checkbox enabled.

[elasticmachine]

Pinging @elastic/security-threat-hunting (Team:Threat Hunting)

[stephmilovic]

should "stack by" change url params as well? @paulewing