Add plugins folder with tools plugin components

[pvaneck]

This adds a plugins directory at the root of the folder with currently just main azure-sdk-tools plugin for copilot/agency/claude.

In the Agency playground repo, we can reference the plugin content from there since they support listing externally sourced plugins.

The benefits of this are:

1. Avoids having to sync with the agency repository.
2. Keeps sdk-tools related content in one place, which makes it easier to maintain and update.
3. Easier to contribute to tools repo vs agency repo.

If we make this the source of truth for skills/agents/etc, then we can remove the .github/skills directory and update the skill-eval.yml workflow to reference the skills here.