[Feature]: add add_token_to_env provider argument

[simonostendorf]

What whould you like to see?

I think it would be helpful to add the token to the env var HCLOUD_TOKEN if not already present.

This would help when using external scripts (e.g. terraform_data resource with local provisioner) that require a Hetzner Cloud API Token.

My usecase: I am checking if a hcloud snapshot exists (using the hcloud_images data source), if not I am calling the hcloud-upload-image binary using a terraform_data resource. So it would be helpful to use the same token as the provider and therefor the token must/should be exported as HCLOUD_TOKEN.

See example add_address_to_env from terraform-provider-vault