plugins: support git-subdir marketplace source kind (#317362)

Adds support for the `git-subdir` plugin source kind in marketplace.json files,
matching the Claude Code marketplace schema. The `git-subdir` source clones an
arbitrary git URL and installs a plugin from a subdirectory of that repository,
useful for monorepos that publish many plugins from a single repo.

- Extends `IGitUrlPluginSource` with an optional `path` field.
- Accepts both `"source": "url"` and `"source": "git-subdir"` discriminants
  when parsing marketplace.json. `git-subdir` requires both `url` and
  `path`; the .git suffix is not required (Azure DevOps / AWS CodeCommit).
- Updates `GitUrlPluginSource` to resolve the install URI as repo-root + path
  (mirroring the existing GitHub sub-path handling), while the clone target
  remains the repository root.
- Includes `path` in update detection (`hasSourceChanged`) and the
  human-readable label (`getPluginSourceLabel`).

Fixes https://github.com/microsoft/vscode/issues/310850

(Commit message generated by Copilot)