ENG-8693: Add value_type support to the Terraform provider

[christopher-bulger]

Closes ENG-8693

This PR adds support to specify value_type when creating a secret with Terraform.

resource "doppler_secret" "db_password" {
  project = doppler_project.backend.name
  config = doppler_config.backend_stg.name
  name = "DB_PASSWORD"
  value = "1"
  value_type = "integer"
}

[nmanoogian]

In addition to adding this field to the resourceSecretUpdate func, we need to add it to resourceSecretRead to detect out-of-band changes to the secret value type. Unfortunately, the GET /v3/configs/config/secret endpoint that we use today doesn't have this data available. We can discuss in ENG-8693 how to go about adding it