chore: Add CODEOWNERS file (#187)

Author: farleyb-amazon

Diff for: .github/CODEOWNERS

@@ -0,0 +1,5 @@
+# Each line is a file pattern followed by one or more owners.
+# https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-code-owners
+
+# Default code owner for everything is our aws-crypto-tools group
+*       @aws/aws-crypto-tools