[Fleet] Flesh out HTTP API around Fleet Actions

[kpollich]

WIP

Create a set of HTTP endpoints around an actions resource that supports:

• Create bulk actions
• Read the status of an action
• Cancel a pending action

Ref #130259

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[joshdover]

The generic actions functionality should also support the same UI workflows that upgrades support, including the rollout status callout on Fleet agents list, with the ability to abort/cancel a rollout.

cc @juliaElastic within the context of the RBAC evolution, we’ve discussed having an actions JS service in order to enforce consistent access control. I don’t have any direct ask, but just want to make sure you’re aware of this effort.