* Upgrade to new Cloud KMS client libraries
We recently released the new Cloud KMS client libraries which use GRPC
instead of HTTP. They are faster and look nicer (</opinion>), but more
importantly they drastically simplify a lot of the logic around client
creation, encryption, and decryption. In particular, we can drop all the
logic around looking up credentials and base64-encoding/decoding.
Tested on a brand new cluster (no pre-existing unseal keys) and against
a cluster with stored keys from a previous version of Vault to ensure no
regressions.
* Use the default scopes the client requests
The client already does the right thing here, so we don't need to
surface it, especially since we aren't allowing users to configure it.
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