Add envoy proxy icon

[AndrooTheChen]

Description

A buildkite icon for Envoy proxy (https://www.envoyproxy.io/)

PR checklist

• [x] The image is 64x64 PNG image
• [x] Image is added to img-buildkite-64 directory
• [x] Image is referenced in img-buildkite-64.json file
• [x] Image is referenced in README markdown file
• [x] The image can be hosted by Buildkite and made available in Buildkite UI