In very rare cases, we could start an RPC before the picker had been updated to
one that would return a valid SubConn. This is not a problem as the new picker
will be called again as soon as it is updated, but it can lead to test flakes
that depend upon the picker not being called before being ready.
Google default creds is a combo of ALTS, TLS and OAuth2. The right set of creds will be picked to use based on environment.
This PR contains:
- A new `creds.Bundle` type
- changes to use it in ClientConn and transport
- dial option to set the bundle for a ClientConn
- balancer options and NewSubConnOption to set it for SubConn
- Google default creds implementation by @cesarghali
- grpclb changes to use different creds mode for different servers
- interop client changes for google default creds testing
