Mechanism to automatically scan for and identify out-of-date/non-maintained extensions installed by default

[gfmatthews]

The problem: Most (if not all) container definitions include extensions from the VS marketplace to enable core scenarios like per-test run/debug, syntax highlighting, etc. If one of these extensions becomes abandoned, or archived (as was the case in #1253 ), maintainers of the images won't get automatically notified.

Proposal: Include, as part of scheduled automated CI runs, a report that iterates through all the manifests in this repository and looks for extensions that are out of date or should be otherwise retired.

Some signals we could look for (input requested):

• Extension not updated in more than <X> days
• Extension's source code is marked as archived

Relates to: Both

[bamurtaugh]

cc @joshspicer @chrmarti for thoughts