Class ChallengeHandler

Handles challenge based authentication for Container Registry Service.

 The challenge-based authorization flow for ACR is illustrated in the following steps.
 For example, GET /api/v1/acr/repositories translates into the following calls.
     Step 1: GET /api/v1/acr/repositories
 Return Header: 401: www-authenticate header - Bearer realm="{url}",service="{serviceName}",scope="{scope}",error="invalid_token"
     Step 2: Retrieve the serviceName, scope from the WWW-Authenticate header.  (Parse the string.)
     Step 3: POST /api/oauth2/exchange
 Request Body : { service, scope, grant-type, aadToken with ARM scope }
 Response Body: { acrRefreshToken }
     Step 4: POST /api/oauth2/token
 Request Body: { acrRefreshToken, scope, grant-type }
 Response Body: { acrAccessToken }
     Step 5: GET /api/v1/acr/repositories
 Request Header: { Bearer acrTokenAccess }