3.0.0-beta02 of all GAX packages

Release GAX 3.0.0-beta02

Changes since 3.0.0-beta01:

- Google.Apis.Auth dependency updated
- The "gccl" value in x-goog-api-client is now omitted
- ServiceSettingsBase exposes VersionHeaderBuilder publicly so that manual libraries can add "gccl" back into x-goog-api-client
- The gRPC ClientBuilderBase respects cancellation tokens more widely
- ChannelPool and ScopedCredentialProvider methods for retrieving
  channels/credentials have been made internal, as they're now not
  used within API libraries, only in ClientBuilderBase