Introduce ability for OAuth Secrets to be imported from environment variables

[kaovd]

Implements functionality described in #307

Adds additional test to reflect checks

Have tested a local build and found it works fine

Correct configuration:

Erroneous configuration where environment var does not exist:

[DRuggeri]

Hey there, @kaovd - thanks for the pull request! I'm looking at this change and am curious about the use case. I think this is already supported natively in terraform through two existing options:
erraform already supports environment variables natively through several mechanisms:

Option 1: Terraform Variables

variable "oauth_client_id" {
  type      = string
  sensitive = true
}

variable "oauth_client_secret" {
  type      = string
  sensitive = true
}

provider "restapi" {
  uri = "https://graph.microsoft.com"
  
  oauth_client_credentials {
    oauth_client_id      = var.oauth_client_id
    oauth_client_secret  = var.oauth_client_secret
    oauth_token_endpoint = "https://login.microsoftonline.com/..."
  }
}

Then set the values via environment variables (the strings can be variable interpolated from your shell, too):

export TF_VAR_oauth_client_id="your-client-id"
export TF_VAR_oauth_client_secret="your-secret"

Option 2: Using Terraform's env() function (TF 1.2 needed)

provider "restapi" {
  uri = "https://graph.microsoft.com"
  
  oauth_client_credentials {
    oauth_client_id      = env("MY_OAUTH_CLIENT_ID")
    oauth_client_secret  = env("MY_OAUTH_CLIENT_SECRET")
    oauth_token_endpoint = "https://login.microsoftonline.com/..."
  }
}

Is there something missing in the native capabilities that this would close out?