[Cases] Add a quick link to the external service incident in the connectors panel

[cnasikas]

To navigate to the external service incident (pushed case) the user has to either click on the link in the user activity or click on the case actions and then click on the link (View <incident name>). The work in #140096 will filter and paginate the user activity so the link may be hidden. The only alternative is to go to the case actions. Still, it is not apparent (or easily accessible) where the URL to the external service is. For this reason, we should show the link to the connectors panel. This work can be combined with the work in #117904.

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)