Device Defender Custom Metrics Support

Adds support for Device Defender Custom Metrics and adds a new Device Defender sample! Also adds some fixes to the Shadow sample, documentation generation updates, and more.

What's Changed

• Added ClientToken to ShadowDeltaUpdated by @TwistedTwigleg in #417
• Autodocs by @graebm in #419
• Update Shadow sample to get value before input by @TwistedTwigleg in #420
• Device Defender custom metrics support by @TwistedTwigleg in #401
• Updated PreRequisites file by @TwistedTwigleg in #421
• Readme Install Instructions Update by @TwistedTwigleg in #426
• Added note on supported data requirements for Device Defender by @TwistedTwigleg in #429
• Fix segmentation fault in iot client init functions by @TwistedTwigleg in #430
• Minor sample adjustments by @TwistedTwigleg in #432

Full Changelog: v1.17.0...v1.18.0