* plumb context to keystore methods
Contexts are more relevant now with the GCP KMS keystore which makes
http requests, this commit makes sure they are passed everywhere they
are needed, with only a few `context.TODO`s sprinkled in.
* feat: add GCP KMS support for Teleport CA key material
This commit implements support for GCP KMS as a backend for CA
operations in Teleport.
This is able to take advantage of much of the infrastucture that we have
already created for HSM support, and simply appears as a new backend for
the private key material.
The necessary configuration parameters include only the name of the KMS
keyring to use, and the protection level (which can be HSM or SOFTWARE).
These are configured in the teleport.yaml directly, in a new section
under the existing `ca_key_params` used for HSM configuration.
The GCP credentials are expected to be provided to the Teleport auth
server via the
[Application Default Credentials](https://cloud.google.com/docs/authentication/provide-credentials-adc).
This means that it "just works" if the auth server is running on a GCP
compute instance with the correct attached role, and you can run tests
locally by authenticating with `gcloud auth login`.
This does not support Teleport Cloud, as our current HSM support does
not, because the Auth server needs the configuration and the access to
the GCP account. That would be a larger effort probably requiring a new
Teleport service.
* shrink/remove keystore interface
This commit introduces the keystore.Manager type to handle all
interaction between CA and the keystore backends.
Why:
* reduces the code that needs to be implemented per keystore backend to
only the necessary operations
* separate concerns of managing key material and handling CA data
structures
* define interfaces where they're used, not implemented
* delete net 245 lines of code
* reduce keystore.KeyStore stutter
Nic Klaassen