Fix invalid config schema for metis.settings configuration

The configuration schema in metis.schema.yml is not defined under the `metis.settings` config object. Instead, top-level keys such as `metis_default_server` and `metis_force_ssl` are declared directly.

This does not match current Drupal config schema expectations and should be corrected before calling the module Drupal 11 compatible.

Current problem areas:

- `metis_default_server`, `metis_force_ssl`, `username`, and `password` are missing the `metis.settings` parent schema.
- The file should define `metis.settings` as a `config_object` with a `mapping`.

Proposed change:

- Replace the top-level scalar entries with a proper schema definition for `metis.settings`.
- Keep `field.value.metis` as a separate schema entry for field values.

Suggested acceptance criteria:

- Drupal recognizes `metis.settings` without schema warnings.
- Config schema validation passes on Drupal 10/11.