fix(variable): change 'value' type to Dynamic

[mitchnielsen]

Summary

Changes the Variable 'value' type from string to Dynamic.

In PrefectHQ/prefect#13543 and associated changes around May 2024, Variables were updated from simple strings to JSON objects. The Terraform provider has still been treating them as strings, so when folks tried to put JSON-compatible values in them, Terraform would fail to work with them as found in #254

That said, this doesn't mean you can always expect a full JSON object in response. It could be individual JSON-compatible types, like bool or int. So we actually need a Dynamic attribute type here.

Related to https://linear.app/prefect/issue/PLA-247/changing-variable-to-a-json-value-in-the-ui-makes-next-terraform-run

Related to #254

References

From https://docs.prefect.io/3.0/develop/variables:

(Variables) are named, mutable values of any JSON type

Also, you can see the accepted types for the value field here:

$ curl -s https://api.prefect.cloud/api/openapi.json | jq '.components.schemas.Variable.properties.value.anyOf'
$ curl -s https://api.prefect.cloud/api/openapi.json | jq '.components.schemas.Variable.properties.value.anyOf[]'
{
  "type": "string",
  "maxLength": 5000
}
{
  "type": "integer"
}
{
  "type": "number"
}
{
  "type": "boolean"
}
{
  "type": "object"
}
{
  "items": {},
  "type": "array"
}

[mitchnielsen]

So changing this to a JSON type may not be the full fix here. As the API response shows, the value here can be almost anything: string, integer, number, boolean, object, and array. So if I put 5 in the Variable, the provider will fail to convert that into map[string]interface. Need to think about this some more.

[mitchnielsen]

We may need to use a dynamic attribute here: https://developer.hashicorp.com/terraform/plugin/framework/handling-data/attributes/dynamic