Add option to sign using HMAC-SHA-256

[fabio-looker]

Via the api, secrets can now be created that are bound to being used with HMAC-SHA-256 instead of HMAC-SHA-1. Add a feature to allow users to test these secrets.

https://docs.looker.com/reference/api-and-integration/api-reference/v4.0/auth