Support client-redirection

When connecting to the secondary in a high availability setup, it is possible to get a `307 TEMPORARY REDIRECT` response when Vault cannot forward a request to the primary on behalf of the client ([docs](https://www.vaultproject.io/docs/concepts/ha.html#client-redirection)).

We currently don't follow these redirects and might want to support this.

Start by locating the Haskell entry point that connects to Vault and sends requests when the client is connected to a secondary. Read the linked Vault HA client-redirection documentation and inspect the current handling of HTTP responses; done means the 307 case is handled as intended and covered by an appropriate test.

Written by the indexing model from the issue text.