chore: add Josh Medeski as `wp-cli` plugin maintainer

.github/CODEOWNERS

@@ -3,3 +3,4 @@ plugins/aws/                        @maksyms
 plugins/git-lfs/                    @vietduc01100001
 plugins/gitfast/                    @felipec
 plugins/sdk/                        @rgoldberg
+plugins/wp-cli/                     @joshmedeski