[passport-oauth2] Allows callbackURL option to be omitted in constructor of OAuth2Strategy (#28907)

* [passport-oauth2] callbackURL is an optional value

The implementation for this OAuth2Strategy handles `callbackURL` being undefined, so this more accurately describes the interface.

* Making at least one test case use no callbackURL

types/passport-oauth2/index.d.ts

@@ -45,7 +45,7 @@ declare namespace OAuth2Strategy {
         tokenURL: string;
         clientID: string;
         clientSecret: string;
-        callbackURL: string;
+        callbackURL?: string;
     }
     interface StrategyOptions extends _StrategyOptionsBase {
         passReqToCallback?: false;

types/passport-oauth2/passport-oauth2-tests.ts

@@ -33,7 +33,6 @@ function verifyFunction4(_req: Request, _accessToken: string, _refreshToken: str
 
 const strategyOptions2: StrategyOptionsWithRequest = {
     authorizationURL: 'http://www.example.com/auth',
-    callbackURL: 'http://www.example.com/callback',
     clientID: 'dummy',
     clientSecret: 'secret',
     tokenURL: 'http://www.example.com/token',