Adds `UpsertCertAuthority` to the `teleport.v1.trust.TrustService`
and deprecates the HTTP api. The `services.Trust` interface was
also updated to include a `context.Context` in both
`CreateCertAuthority` and `UpsertCertAuthority.
To ensure backwards compatibility the `auth.Client` will first
attempt to use the gRPC client and if that fails with a not
implemented error it will fallback to using HTTP.
Part of #6394