Implement ImportState and more robust Update checks

[RaynorChavez]

• We implement ImportState so users can import existing resources created outside of Terraform IaC
• Also add checks for updating fields - if you try to update any field other than inference type, number of inferences, shards, and replicas, then we fail fast instead of sending the request to marqo only for it to fail after the operation is done

[orlade]

Do you really need reflect.DeepEqual to check whether indexDetail.TextPreprocessing is an empty struct of primitives? Can you just if indexDetail.TextPreprocessing != (go_marqo.TextPreprocessing{}) {...?

Also, nit, it would be marginally more readable to swap the conditions, i.e. if indexDetail.TextPreprocessing == (go_marqo.TextPreprocessing{}) {model.Settings.TextPreprocessing = nil} else {...unpacking stuff...}

[RaynorChavez]

Good idea thanks

[orlade]

Isn't this already nil by default? Do you need to set it explicitly?

[RaynorChavez]

We need to explicitly set it here to normalize the value for ModelProperties in all possible situations that might lead to its value becoming an empty dict {}

So for example

1. API might return an empty dict (we don't but if we ever did it would get corrected here)
2. initial state - If a user specifies empty model properties (say {}) in their configuration, the state should show nil rather than an empty object.
3. Refresh - need to normalize 2 (the config file) everytime we refresh