Add @shalier as a codeowner maintainer (openservicemesh#5261)

@shalier has been contributing to the project at the level that is
expected of a maintainer.

Maintainers, if you agree with this nomination, leave a "+1" comment
and approve the PR (do not merge it).

Signed-off-by: Thomas Stringer <thomas@trstringer.com>

OWNERS

@@ -6,6 +6,7 @@
 - @trstringer
 - @keithmattix
 - @steeling
+- @shalier
 
 # Non-code maintainers
 - @phillipgibson