Add keithmattix as a Policies and Telemetry maintainer

[keithmattix]

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:

• https://github.com/envoyproxy/envoy/pull/35074
• https://github.com/envoyproxy/envoy/pull/33857
• https://github.com/envoyproxy/envoy/pull/33362
• https://github.com/envoyproxy/envoy/pull/32961
• https://github.com/istio/proxy/pull/5617
• https://github.com/istio/proxy/pull/5514

[kfaseela]

Congrats 👏

[keithmattix]

/retest