[Alerting] Rules should handle actions where connectors have isMissingSecrets: true

[ymao1]

In order to support import/export of connectors, we will be adding the ability to disable connectors after import and we need to ensure that rules can handle actions using disabled connectors.

1. Rules shouldn't schedule actions if connector is disabled

Rules already check whether an action is executable before scheduling actions using the isActionExecutable fn. Currently that only checks whether the action is preconfigured or whether the action type is disabled. This fn would need to be updated to also check whether the connector referenced by the action is has isMissingSecrets set to true.

2. Create/edit rule flyout should handle disabled connectors

If an action is using a broken connector (isMissingSecrets: true) or user selects a broken connector for an action, there should be some sort of indicator that the connector is broken. I'm not sure if there should be a link to the connector page to enable the connector since clicking it would navigate the user away from the rule flyout and they'd lose what changes they have made.

[ymao1]

If an action is using a disabled connector or user selects a disabled connector for action, there should be some sort of indicator that the connector is disabled.

There's a question of whether we should prevent users from selecting disabled connectors. In my opinion, I don't think we should prevent them from selecting this, just warn them that it's disabled. This would align better with imported rules with imported disabled connectors as well.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[ymao1]

Blocked by #94106

[ymao1]

PR for #94106 is open so removing blocked label since you can branch off the PR