[core-rest-pipeline] make CAE policy options' credential optional (Azure#15134)

Azure Container Registry allows anonymous pull access, in which case,
TokenCredential is not required for its challenge-based auth
flow. This PR makes credential optional in the bearer token
challenge auth policy options so that an undefined value is allowed.

ACR SDK library would send an unauthorized initial request to ACR Api
endpoint, get a challenge back then continue to handle the challenge.

Author: jeremymeng

sdk/core/core-rest-pipeline/review/core-rest-pipeline.api.md

@@ -64,7 +64,7 @@ export const bearerTokenChallengeAuthenticationPolicyName = "bearerTokenChalleng
 // @public
 export interface BearerTokenChallengeAuthenticationPolicyOptions {
     challengeCallbacks?: ChallengeCallbacks;
-    credential: TokenCredential;
+    credential?: TokenCredential;
     scopes: string[];
 }
 

sdk/core/core-rest-pipeline/src/policies/bearerTokenChallengeAuthenticationPolicy.ts

@@ -76,7 +76,7 @@ export interface BearerTokenChallengeAuthenticationPolicyOptions {
   /**
    * The TokenCredential implementation that can supply the bearer token.
    */
-  credential: TokenCredential;
+  credential?: TokenCredential;
   /**
    * The scopes for which the bearer token applies.
    */
@@ -136,7 +136,9 @@ export function bearerTokenChallengeAuthenticationPolicy(
   // The options are left out of the public API until there's demand to configure this.
   // Remember to extend `BearerTokenChallengeAuthenticationPolicyOptions` with `TokenCyclerOptions`
   // in order to pass through the `options` object.
-  const cycler = createTokenCycler(credential /* , options */);
+  const getAccessToken = credential
+    ? createTokenCycler(credential /* , options */).getToken
+    : () => Promise.resolve(null);
 
   return {
     name: bearerTokenChallengeAuthenticationPolicyName,
@@ -157,7 +159,7 @@ export function bearerTokenChallengeAuthenticationPolicy(
       await callbacks.authorizeRequest({
         scopes,
         request,
-        getAccessToken: cycler.getToken
+        getAccessToken
       });
 
       let response: PipelineResponse;
@@ -179,7 +181,7 @@ export function bearerTokenChallengeAuthenticationPolicy(
           scopes,
           request,
           response,
-          getAccessToken: cycler.getToken
+          getAccessToken
         });
 
         if (shouldSendRequest) {