Add support for custom check in header checker

This allows derived classes to override customCheck method to inject
additional checks for some files. This is currently needed to accept
ARM copyright in ARMv8-M files - FreeRTOS/FreeRTOS-Kernel#1147.

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>

Author: aggarg

.github/scripts/common/header_checker.py

@@ -216,6 +216,10 @@ def isValidFile(self, path):
                 print("PASS")
                 print("-" * 85)
                 return True
+            elif self.customCheck(path):
+                print("PASS")
+                print("-" * 85)
+                return True
             elif self.isThirdPartyFile(path):
                 print("FAIL")
                 print("-" * 85)
@@ -435,3 +439,6 @@ def validateSpdxLine(self, lines):
             else:
                 error_count += 1
         return error_count == 0
+
+    def customCheck(self, path):
+        return False