Add .gitattributes (#4799) (#4808)

This commit adds a .gitattributes file to
treat all shell scripts and .ci/run-tests as LF eol

Co-authored-by: Russ Cam <russ.cam@elastic.co>

Author: github-actions[bot]

.gitattributes

@@ -0,0 +1,4 @@
+* text=auto
+
+*.sh text eol=lf
+.ci/run-tests text eol=lf