fix: double encoding in registry creds

[johanneswuerbach]

There is currently when using a humanitec_registry resource with creds.

With a definition like:

resource "humanitec_registry" "my" {
  id        = "my"
  type      = "amazon_ecr"
  creds = {
    username = "aws"
    password = "key"
  }
}

The previous implementation would send a quoted string the API.

{ "creds": { "username": "\"aws\"", "password": "\"key\"" } }

Fix this to be properly encoded strings.

{ "creds": { "username": "aws", "password": "key" } }

[mateuszjenek]

Nice catch! LGTM!