Build the installer skill in the core plugin for plugin discovery

**Standalone work item**

## Context
> ... to increase their visibility, we will add a new "installer skill" in the core plugin. This skill will have the knowledge of all the plugins in azure-skills repo to help the user search for skills that can help them in various scenarios.

## Scope
- Add an installer skill to the core `azure` plugin.
- Skill knows all plugins in `azure-skills` and helps users find a skill by scenario.

## Acceptance criteria
- Prompting e.g. "I want to do X in Azure, find a skill that can help" surfaces the relevant plugin/skill.