Create settings for extensions within the settings modal

[joadoumie]

Description of the new feature / enhancement

This is an additional part of what is required in #80 for the extensions specifically.

Extensions should be able to host their own settings to help determine how the extension will behave.

Scenario when this would be used?

When extensions want to author settings and when users want to change settings for an extension

Tasks

• Spec out how extensions provide settings to us #122
• Write Settings helpers to abstract away working with settings  #123

[zadjii-msft]

(I'm tagging this up as the third of the work specifically for "the palette can render things spec'd by #122 and implemented in the extensions with #123"). We still need to do the work to link to these settings, if an app provides them.