Fixes #4360. This adds a `--base` option to `snapcraft extensions`, matching the existing `snapcraft plugins --base` behaviour. With this change, users can list only the extensions supported by a specific base, for example: snapcraft extensions --base core24 Changes: - add `--base` to the `extensions` command - filter listed extensions by supported base when provided - reject unsupported and maintenance bases - add unit coverage for base filtering and errors Validation: - `pytest -q tests/unit/commands/test_list_extensions.py -rs` - `ruff check snapcraft/commands/extensions.py tests/unit/commands/test_list_extensions.py`