Introduce plugins support in BC-Bench

[Copilot]

This PR is a prototype by Copilot, not verified.

The goal: Extends experiment configuration to install Copilot/Claude plugins, it should work across both Copilot CLI and Claude Code, with marketplaces pinned to a specific commit for reproducibility.

Config schema

TBD, prototype by coding agent.

plugins.install is a list of entries, each pinning a marketplace to a SHA:

plugins:
  install: []
    # - repo: "github/awesome-copilot"   # marketplace OWNER/REPO or git URL
    #   sha: "28c3a14af4e6232091071ddb40272f72d9d96b2f"
    #   name: "awesome-copilot@awesome-copilot"  # <plugin>@<marketplace>

Goal

BC-Bench must support experimentation with plugins (hosted in a public GitHub repo). In the https://github.com/microsoft/BC-Bench/blob/main/src/bcbench/agent/shared/config.yaml, there should be an entry for plugins.

By default, no plugins are enabled. And there should be a sample one (commented out) for folks to easily test out.

The orchestration should be handled by coding agents (e.g. Copilot CLI) as much as possible => less code here in BC-Bench.

Testing

Before merging, this must be tested (using a public plugin from GitHub maybe):

1. Locally using the sample
2. Tested with few entries, using both GitHub Copilot and Claude Code
3. A full test run in Code Review category, and observe the Kusto storage is correct uploaded