feat: add plugin-directory input to e2e-version

[sunker]

Summary

Adds a plugin-directory input to the e2e-version action so it can find src/plugin.json in the correct location for plugins that live in a subdirectory (e.g. monorepos).

Without this, the action always reads grafanaDependency from the repo root's src/plugin.json, ignoring the actual plugin location. The only workaround was to manually set grafana-dependency, which is easy to forget and gets stale.

Changes

• action.yml: add optional plugin-directory input
• index.js: read plugin.json from plugin-directory when set
• README.md: document the new input

Usage

- uses: grafana/plugin-actions/e2e-version@e2e-version/vX.Y.Z
  with:
    version-resolver-type: plugin-grafana-dependency
    plugin-directory: path/to/plugin