…lastic#65397)

resolves elastic#63171

Previously, preconfigured actions were specified as an array of action
properties.  This ended up being problematic when using the kibana keystore
for secrets, as you'd have to reference specific actions via index.

This changes preconfigured actions to be specified as an object, where the
property key is the id, and the body is the remainder of the action properties.

As access to preconfigured actions has leaked across the code base, it's
probably time to consider changing the internal representation from an array
to a Map, to provide easier access by action id.  For a future PR.