Add a Streamlink plugin (plugin/dashdrm_mp4ff.py) to support DRM-protected MPEG-DASH streams with optional mp4ff-decrypt integration. Include README with usage and installation notes, an interactive helper (gen.py) that reads vault.json and prints Streamlink commands, a vault.json example, pyproject.toml (declares inquirerpy dependency), .python-version, and .gitignore. This provides the plugin, packaging metadata, and tooling needed to run and test DASH-DRM streams locally.
