grpc: support full set of Google credentials. (#3577)
grpc: support full set of Google credentials. (#3577)

This supports things like OAuth, GCE default creds, refresh tokens, etc.

Risk Level: Low
Testing: Minimal tests. grpc::Channel/CallCredentials are opaque and don't provide much in the way
  of inspection from tests (CC: @vjpai).

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 2426ec1c614399438fae6803c85d22b98b1a7038

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api