Add bearerToken constructor to Authentication class

Also added `authorizationHeaderValue()` function.
Also added `const` qualifier to `basic` constructor.

Author: kevmoo

lib/src/common/github.dart

@@ -368,12 +368,9 @@ class GitHub {
       headers['Accept'] = preview;
     }
 
-    if (auth.isToken) {
-      headers.putIfAbsent('Authorization', () => 'token ${auth.token}');
-    } else if (auth.isBasic) {
-      final userAndPass =
-          base64Encode(utf8.encode('${auth.username}:${auth.password}'));
-      headers.putIfAbsent('Authorization', () => 'basic $userAndPass');
+    final authHeaderValue = auth.authorizationHeaderValue();
+    if (authHeaderValue != null) {
+      headers.putIfAbsent('Authorization', () => authHeaderValue);
     }
 
     // See https://docs.github.com/en/rest/overview/resources-in-the-rest-api?apiVersion=2022-11-28#user-agent-required

lib/src/common/util/auth.dart

@@ -1,3 +1,5 @@
+import 'dart:convert';
+
 /// Authentication information.
 class Authentication {
   /// OAuth2 Token
@@ -9,26 +11,65 @@ class Authentication {
   /// GitHub Password
   final String? password;
 
+  final String? bearerToken;
+
+  // TODO: mark the pram as `String` to REQUIRE a non-null value.
+  // NEXT major version
   /// Creates an [Authentication] instance that uses the specified OAuth2 [token].
   const Authentication.withToken(this.token)
       : username = null,
-        password = null;
+        password = null,
+        bearerToken = null;
+
+  /// Creates an [Authentication] instance that uses the specified
+  /// [bearerToken].
+  const Authentication.bearerToken(String this.bearerToken)
+      : username = null,
+        password = null,
+        token = null;
 
   /// Creates an [Authentication] instance that has no authentication.
   const Authentication.anonymous()
       : token = null,
         username = null,
-        password = null;
+        password = null,
+        bearerToken = null;
 
+  // TODO: mark the `username` and `password` params as `String` to REQUIRE
+  // non-null values. - NEXT major version
   /// Creates an [Authentication] instance that uses a username and password.
-  Authentication.basic(this.username, this.password) : token = null;
+  const Authentication.basic(this.username, this.password)
+      : token = null,
+        bearerToken = null;
 
   /// Anonymous Authentication Flag
-  bool get isAnonymous => !isBasic && !isToken;
+  bool get isAnonymous => !isBasic && !isToken && !isBearer;
 
   /// Basic Authentication Flag
   bool get isBasic => username != null;
 
   /// Token Authentication Flag
   bool get isToken => token != null;
+
+  // This instance represents a authentication with a "Bearer" token.
+  bool get isBearer => bearerToken != null;
+
+  /// Returns a value for the `Authorization` HTTP request header or `null`
+  /// if [isAnonymous] is `true`.
+  String? authorizationHeaderValue() {
+    if (isToken) {
+      return 'token $token';
+    }
+
+    if (isBasic) {
+      final userAndPass = base64Encode(utf8.encode('$username:$password'));
+      return 'basic $userAndPass';
+    }
+
+    if (isBearer) {
+      return 'Bearer $bearerToken';
+    }
+
+    return null;
+  }
 }