gitattributes: json and yaml files LF line endings

Always check-out / check-in json and yaml files with LF line endings.

This will also solve a few unit tests on Windows, in which the expectation is that marshalled
objects match the the content written in a few yaml or json files. On Windows, these files
may be checked out with CRLF line endings, which causes the tests to fail.

.gitattributes

@@ -1,3 +1,6 @@
+# Always check-out / check-in files with LF line endings.
+* text=auto eol=lf
+
 hack/verify-flags/known-flags.txt merge=union
 test/test_owners.csv merge=union