Ignore AWS Serverless Application Model build folder (#3357)

https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-cli-command-reference-sam-build.html

community/AWS/SAM.gitignore

+# gitignore template for AWS Serverless Application Model project
+# website: https://docs.aws.amazon.com/serverless-application-model
+
+# Ignore build folder
+.aws-sam/