actions: validate that action referenced in action_trigger exists in config during transform

[mildwonkey]

I misconfigured a test and had the sudden realization that certain validations had gotten lost - crucially, we were no longer validating that the action referenced inside a resource's ActionTriggers existed inside the config. Very early on, that happened during configload, but it was based on flawed implementation (my mistake! what is a terraform, even?) and the validation was removed when I fixed my improper handling.

I tried a few options but decided that this particular validation fit best in the configuration transformation process, as it is a matter of connecting the resource's action to the action configuration. I took some inspiration from validateImportTargets code, mostly in how I crafted the error, and in deciding to return after validating the actions if an error is encountered. I'm open to alternate suggestions!
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1.14.x

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