auth: Allow private key for apps to be base64

Some CI systems have trouble with new lines so encoding them to base64
is standard. This option allows the key to be passed encoded.

post/clang_tidy_review/clang_tidy_review/__init__.py

@@ -4,6 +4,7 @@
 # See LICENSE for more information
 
 import argparse
+import base64
 import fnmatch
 import itertools
 import json
@@ -63,13 +64,23 @@ def add_auth_arguments(parser: argparse.ArgumentParser):
     # Token
     parser.add_argument("--token", help="github auth token")
     # App
-    group_app = parser.add_argument_group("github app installation auth")
+    group_app = parser.add_argument_group(
+        """Github app installation authentication
+Permissions required: Contents (Read) and Pull requests (Read and Write)"""
+    )
     group_app.add_argument("--app-id", type=int, help="app ID")
     group_app.add_argument(
         "--private-key", type=str, help="app private key as a string"
     )
     group_app.add_argument(
-        "--private-key-file-path", type=str, help="app private key .pom file path"
+        "--private-key-base64",
+        type=str,
+        help="app private key as a string encoded as base64",
+    )
+    group_app.add_argument(
+        "--private-key-file-path",
+        type=pathlib.Path,
+        help="app private key .pom file path",
     )
     group_app.add_argument("--installation-id", type=int, help="app installation ID")
 
@@ -80,11 +91,13 @@ def get_auth_from_arguments(args: argparse.Namespace) -> Auth:
 
     if (
         args.app_id
-        and (args.private_key or args.private_key_file_path)
+        and (args.private_key or args.private_key_file_path or args.private_key_base64)
         and args.installation_id
     ):
         if args.private_key:
             private_key = args.private_key
+        elif args.private_key_base64:
+            private_key = base64.b64decode(args.private_key_base64).decode("ascii")
         else:
             private_key = open(args.private_key_file_path).read()
         return Auth.AppAuth(args.app_id, private_key).get_installation_auth(
@@ -94,11 +107,12 @@ def get_auth_from_arguments(args: argparse.Namespace) -> Auth:
         args.app_id
         or args.private_key
         or args.private_key_file_path
+        or args.private_key_base64
         or args.installation_id
     ):
         raise argparse.ArgumentError(
             None,
-            "--app-id, --private-key[-file-path] and --installation-id must be supplied together",
+            "--app-id, --private-key[-file-path|-base64] and --installation-id must be supplied together",
         )
 
     raise argparse.ArgumentError(None, "authentication method not supplied")