Features to remove for V2

[bshaffer]

This is a list of items we want to change / remove in the next major release

• Remove all instances of getLastReceivedToken
• Remove all instances of tokenCallback
• The constructor of IAM should not contain $httpHandler - this should be passed in at runtime
• Consider removing IAM in favor of using IamTrait in the credentials which use it.
• Remove FetchAuthTokenCache in favor of CacheTrait
• Consider adding $headers as second argument to FetchAuthTokenIterface::fetchAuthToken, or consolidate everything to use updateMetadata
• Consider consolidating FetchAuthTokenInterface, UpdateMetadataInterface, ProjectIdProviderInterface, GetQuotaProjectInterface, and GetUniverseDomainInterface into a CredentialInterface.
• Consider removing HttpClientCache, as $httpHandler can already be passed in everywhere
• Remove Guzzle6HttpHandler
• Remove IAMCredentials
• Move to Firebase JWT for token validation (https://github.com/googleapis/google-auth-library-php/pull/554)