Add UI dashboard template URL placeholder field: Service.Namespace

#### Feature Description

Add the following placeholders to interpolation for [`ui_config.dashboard_url_templates`](https://www.consul.io/docs/agent/options#ui_config_dashboard_url_templates):
- `Service.Namespace` - not yet implemented, even though they are mentioned in the docs
- partition (once available)

See #11321 for more details on this field.

Trace the interpolation path for ui_config.dashboard_url_templates and compare its supported placeholders with the linked documentation. Read #11321 for the field details, then determine how Service.Namespace and the future partition placeholder should be represented; done means the documented placeholder is interpolated correctly and the relevant behavior is covered by verification in the existing subsystem.

Written by the indexing model from the issue text.