Add static people list for new Adaptive Card People Picker

The Adaptive Card team is adding a new "People Picker" control which will allow the selection of users within a given scope that the card is displayed in (current conversation, org, etc.) To properly test this within the emulator we should add a new setting that lets the developer configure a static list of users that will be searched over when testing the people picker within the emulator.

The time frames for when this new feature rolls is not clear yet but it is coming sometime in 2021.

[enhancement]