Removed google-cloud-auth dependency from other crates.

[yoshidan]

• I made google-cloud-auth optional to use google-cloud-spanner, google-cloud-pubsub and google-cloud-storage.

• I added the google-cloud-token crate.

  • This crate provides TokenSourceProvider, which is minimal trait to get authentication token.
  • To implements TokenSourceProvider makes you to use your own auth crate.
  • google-cloud-auth also implements TokenSourceProvider DefaultTokenSourceProvider

• We can use google-cloud-default to use google-cloud-auth.

  • This crate provides default implementation forClientConfig

Here is the pubsub example to use google-cloud-default.

use google_cloud_default::WithAuthExt;
use google_cloud_pubsub::client::{Client, ClientConfig};

let config = ClientConfig::default().with_auth().await.unwrap();
let client = Client::new(config).await.unwrap();

Other Changes

Spanner

• Removed anyhow crate and merged TxError, InitializationError and RunInTxError into simple Error

ref #92 #99 #101