Improve AwsIotMqtt5ClientBuilder & Samples

What's Changed

• allow direct setting of connect properties to builder by @sbSteveK in #663
• Restore the pubsub samples so we dont break ConnectOneDeviceService by @xiazhvera in #664
• Enhance samples by @sbSteveK in #657
• Improve README by @sfod in #662
• FAQ Update by @xiazhvera in #666
• Add CI to validate required files by @xiazhvera in #665
• AutoTag PR for v1.28.0 by @bretambrose in #669

Full Changelog: v1.27.4...v1.28.0