Decode action blocks and action_triggers inside resource blocks.

[mildwonkey]

This commit adds decoding and merging action and action_triggers inside terraform configuration. The merging behavior is consistent with other terraform objects (the very short version: attributes merge, blocks override). This is likely incomplete, but a good starting point for reviews.

I added an Actions experiment as a somewhat hacky way of keeping the functionality suppressed until we're ready for the alpha; this may never be a formal "experiment" but it's a handy feature-flag-ish-thing so we don't have to do all the work in a long-lived feature branch. (It doesn't matter much at this phase, but once we start adding action functionality this will be important). I will remove experimentsAllowed from the decodeResourceBlock method signature as soon as this is complete.

And for ✨ extra confirmation ✨ that none of this is user-facing, terraform (built from this branch) ignores any action blocks it finds: this works just fine

action "random_action" "hello_world" {
}

resource "random_pet" "meow" {
  lifecycle {
    action_trigger {
        events = [after_create, after_update]
        actions = [action.random_action.hello_world]
    }
  }
}

Terraform will perform the following actions:

  # random_pet.meow will be created
  + resource "random_pet" "meow" {
      + id        = (known after apply)
      + length    = 2
      + separator = "-"
    }

Plan: 1 to add, 0 to change, 0 to destroy.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

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