These enable SDK clients to implement custom request authorization and challenge handling (closes #1756).

New API:

• OnRequest trait: defines a callback invoked on each request, responsible for authorizing it before sending
  • Key Vault clients will use this to skip authorization so they can discover auth parameters from a challenge
• OnChallenge trait: defines a callback invoked upon receiving an auth challenge. Implementations are responsible for parsing challenges and authorizing requests and telling the policy whether to retry them
• BearerTokenAuthorizationPolicy builder methods: with_on_request() and with_on_challenge() set callbacks for a policy instance
  • when these aren't set, the policy behaves as it does today: it authorizes each request with its credential and scopes, and returns 401 responses to the pipeline instead of trying to handle their challenges
• Authorizer trait (sealed): allows callbacks to authorize requests and cache tokens without using the policy's credential directly
  • I moved the authNZ logic from send() into a private implementation of this
• Request::body_mut(): lets the policy reset a body stream before retrying after a challenge

I put all this in a new module azure_core::http::policies::auth and moved BearerTokenAuthorizationPolicy there as well.