Update Greengrass IPC model

What's Changed

• Add macos-latest as optional CI test image by @xiazhvera in #587
• Add Greengrass CI job by @sfod in #581
• add device farm ci job testing on android 8 by @sbSteveK in #589
• Fix CI by @sfod in #591
• Update Greengrass IPC by @sfod in #590
• Bump CRT version to 0.30.0 by @sfod in #592
• AutoTag PR for v1.21.0 by @bretambrose in #593

Full Changelog: v1.20.7...v1.21.0