Update docs for new test/sign params for managed key api and GCP parameters (#17323)
Update docs for new test/sign params for managed key api and GCP parameters (#17323)

* Update docs for new test/sign params for managed key api

 - The existing test/sign managed key api now has two new api params
   allowing an operator to specify to use RSA PSS signatures (use_pss)
   and to specify the hashing algorithm to use (hash_algorithm)

* Remove duplicate GCP signing algo entry

* Formatting nits and mention the key_ring for GCP needs to exist prior to usage

* Add some additional GCP environment vars
hashicorp/vault,hashicorp/vault,hashicorp/vault,hashicorp/vault,hashicorp/vault,hashicorp/vault