add header checker config (GoogleCloudPlatform#5227)

* add header checker config

* add requirements files to ignore

Author: leahecole

.github/header-checker-lint.yml

@@ -0,0 +1,15 @@
+{"allowedCopyrightHolders": [],
+ "allowedLicenses": ["Apache-2.0", "MIT", "BSD-3"],
+ "ignoreFiles": ["requirements.txt", "requirements-test.txt"],
+ "sourceFileExtensions": [
+ 	"ts", 
+ 	"js", 
+ 	"java", 
+ 	"sh", 
+ 	"Dockerfile", 
+ 	"yaml", 
+ 	"py",
+ 	"html",
+ 	"txt"
+ ]
+}