Add @XSAM and @dmathieu as repository maintainers (#5558)

After off-line discussions, the existing maintainers have all agreed
that the contributions made by both @XSAM and @dmathieu motivate adding
them as maintainers to the project. Both @XSAM and @dmathieu have
confimed they are interested in the position and willing to accept the
responsibilities.

### TODO

- [ ] Add @XSAM to maintainers group
- [ ] Add @dmathieu to maintainers group
- [ ] Copy the substance of this PR into the contrib repository

Co-authored-by: Sam Xie <sam@samxie.me>

CODEOWNERS

@@ -14,4 +14,4 @@
 
 * @MrAlias @XSAM @dashpole @MadVikingGod @pellared @hanyuancheung @dmathieu
 
-CODEOWNERS @MrAlias @MadVikingGod @pellared @dashpole
+CODEOWNERS @MrAlias @MadVikingGod @pellared @dashpole @XSAM @dmathieu

CONTRIBUTING.md

@@ -628,15 +628,15 @@ should be canceled.
 
 ### Approvers
 
-- [Sam Xie](https://github.com/XSAM), Cisco/AppDynamics
 - [Chester Cheung](https://github.com/hanyuancheung), Tencent
-- [Damien Mathieu](https://github.com/dmathieu), Elastic
 
 ### Maintainers
 
-- [David Ashpole](https://github.com/dashpole), Google
 - [Aaron Clawson](https://github.com/MadVikingGod), LightStep
+- [Damien Mathieu](https://github.com/dmathieu), Elastic
+- [David Ashpole](https://github.com/dashpole), Google
 - [Robert Pająk](https://github.com/pellared), Splunk
+- [Sam Xie](https://github.com/XSAM), Cisco/AppDynamics
 - [Tyler Yahn](https://github.com/MrAlias), Splunk
 
 ### Emeritus