Add keithmattix as a Policies and Telemetry maintainer

Istio has very few maintainers here and though I'm still learning, I've contributed a handful of upstream envoy PRs as well as 2 recent changes to istio-proxy's telemetry. If existing maintainers have other areas they'd like to see more contributions in, I welcome the feedback!

PRs:
- envoyproxy/envoy#35074
- envoyproxy/envoy#33857
- envoyproxy/envoy#33362
- envoyproxy/envoy#32961
- istio/proxy#5617
- istio/proxy#5514

org/teams.yaml

@@ -189,6 +189,7 @@ teams:
       WG - Policies and Telemetry  Maintainers:
         description: Maintainers for the Policy and Telemetry working group.
         members:
+          - keithmattix
           - kyessenov
           - lei-tang
           - zirain