Improve segment configuration with template engine support

[micpapal]

Context

The current segment configuration uses a simple $group token that gets instantiated once per registered group. While this covers the primary use case (per-group hub-and-spoke isolation), it's limited when more complex segment topologies are needed.

For example, today you can do:

topology:
  segments:
    - name: segment-$group
      links:
        - name: cloud
          neighbors: [$group]

But you cannot express things like:

• "customer-a should see all customer-a-* sub-groups"
• Conditional segment membership based on group name patterns
• Multiple groups matching a prefix in a single segment

Proposal

Replace the simple $group substitution with a proper template engine, similar to what ArgoCD / Helm / External Secrets Operator use. The config would support a segments-template field with Jinja2-style syntax:

topology:
  segments-template: |
    {% for group in groups %}
    - name: segment-{{ group }}
      links:
        - name: cloud
          neighbors: [{{ group }}]
    {% endfor %}

This enables more advanced patterns like prefix-based grouping:

topology:
  segments-template: |
    - name: segment-customer-a
      links:
        {% for group in groups %}
        {% if group is startingwith("customer-a-") %}
        - name: {{ group }}
          neighbors: [customer-a]
        {% endif %}
        {% endfor %}

Possible engine to integrate

MiniJinja (minijinja crate)