Ability to execute an action immediately on the server side

[mikecote]

Currently there is an execute function within the action plugin's start contract but it goes through task manager to execute the task. It would be nice to have the option to make it run right away without queueing in task manager like the HTTP /_execute endpoint does.

cc @XavierM

[elasticmachine]

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

[pmuellr]

Some notes from our triage session (IIRC):

• the current execute API lives on the plugin, not the actionClient, but should probably be on the actionClient - I think with RBAC it will HAVE to move there anyway.
• we probably want to expose two separate APIs on actionClient - the current "queued" executor, which can only return a status of whether the task got queued or not - and an "immediate" one that returns the results of the execution (eg, to get Slack/email/etc error messages).