Adds Xun Zhang (Zhangxunmt) to the maintainers. (#6793)

Signed-off-by: David Venable <dlv@amazon.com>

Author: dlvenable

.github/CODEOWNERS

@@ -1,2 +1,2 @@
 # This should match the owning team set up in https://github.com/orgs/opensearch-project/teams
-*   @divbok @sb2k16 @san81 @srikanthjg @graytaylor0 @dinujoh @kkondaka @KarstenSchnitter @dlvenable @oeyh
+*   @divbok @sb2k16 @san81 @srikanthjg @graytaylor0 @dinujoh @kkondaka @KarstenSchnitter @dlvenable @oeyh @Zhangxunmt

MAINTAINERS.md

@@ -16,6 +16,7 @@ This document contains a list of maintainers in this repo. See [opensearch-proje
 | Karsten Schnitter      | [KarstenSchnitter](https://github.com/KarstenSchnitter)   | SAP         |
 | David Venable          | [dlvenable](https://github.com/dlvenable)                 | Amazon      |
 | Hai Yan                | [oeyh](https://github.com/oeyh)                           | Amazon      |
+| Xun Zhang              | [Zhangxunmt](https://github.com/Zhangxunmt)               | Amazon      |
 
 
 ## Emeritus