agentPlugins: normalize to user data dir storage (#304977)

* agentPlugins: normalize to user data dir storage

Previously we stored plugins in a very internal way that was inaccessible
by other tooling. This sets up a `agent-plugins` folder beside `extensions`
and creates an `installed.json` which is easy to integrate with.

* comments

* fix compile

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Co-authored-by: Raymond Zhao <7199958+rzhao271@users.noreply.github.com>