v2/auth: add Basic authentication and refactor

This adds Basic authentication support.

Along the way, the auth module has been refactored with new enums and
structs to model the supported authentication mechanisms and make the
response header parsing more robust.

***Breaking API changes***

* The `Client::login` and `Client::authenticate` methods have been
  merged into the latter.
* `Client::is_auth` now performs a pure authentication test without
  attempting to authenticate, thus it no longer takes a token.

examples/common/mod.rs

@@ -1,23 +1 @@
 extern crate futures;
-
-pub async fn authenticate_client(
-    mut client: dkregistry::v2::Client,
-    login_scope: String,
-) -> Result<dkregistry::v2::Client, dkregistry::errors::Error> {
-    if !client.is_v2_supported().await? {
-        return Err("API v2 not supported".into());
-    }
-
-    if client.is_auth(None).await? {
-        return Ok(client);
-    }
-
-    let token = client.login(&[login_scope.as_str()]).await?;
-
-    if !client.is_auth(Some(token.token())).await? {
-        Err("login failed".into())
-    } else {
-        println!("logged in!");
-        Ok(client.set_token(Some(token.token())).clone())
-    }
-}

examples/image-labels.rs

@@ -69,7 +69,7 @@ async fn run(
     let login_scope = format!("repository:{}:pull", image);
     let version = dkr_ref.version();
 
-    let dclient = common::authenticate_client(client, login_scope).await?;
+    let dclient = client.authenticate(&[&login_scope]).await?;
     let manifest = match dclient.get_manifest(&image, &version).await {
         Ok(manifest) => Ok(manifest),
         Err(e) => Err(format!("Got error {}", e)),

examples/image.rs

@@ -98,7 +98,7 @@ async fn run(
 
     let login_scope = format!("repository:{}:pull", image);
 
-    let dclient = common::authenticate_client(client, login_scope).await?;
+    let dclient = client.authenticate(&[&login_scope]).await?;
     let manifest = dclient.get_manifest(&image, &version).await?;
     let layers_digests = manifest.layers_digests(None)?;
 

examples/login.rs

@@ -53,7 +53,7 @@ async fn run(
         .password(passwd)
         .build()?;
 
-    let dclient = common::authenticate_client(client, login_scope).await?;
-    dclient.is_v2_supported().await?;
+    let dclient = client.authenticate(&[&login_scope]).await?;
+    dclient.is_auth().await?;
     Ok(())
 }

examples/tags.rs

@@ -57,7 +57,7 @@ async fn run(
 
     let login_scope = format!("repository:{}:pull", image);
 
-    let dclient = common::authenticate_client(client, login_scope).await?;
+    let dclient = client.authenticate(&[&login_scope]).await?;
 
     dclient
         .get_tags(&image, Some(7))

examples/trace.rs

@@ -73,7 +73,7 @@ async fn run(
 
     let login_scope = "";
 
-    let dclient = common::authenticate_client(client, login_scope.to_string()).await?;
+    let dclient = client.authenticate(&[&login_scope]).await?;
     let manifest = dclient.get_manifest(&image, &version).await?;
 
     let layers_digests = manifest.layers_digests(None)?;