Add templating support for dynamic test generation

[Copilot]

Extends the test schema to support "dynamic tests" — tests that expand into multiple instances via the cross-product of template variable values, eliminating repetitive test definitions for things like URI or payload word lists.

Schema changes

• New TemplateDefinition type — defines a named template variable (key) and its possible values
• templates field on Test — optional list of TemplateDefinition entries; when present, the test is expanded into N×M×… instances (one per combination)
• Input fields may reference template variables using Go template syntax: {{ .varname }}

tests:
  - test_id: 1
    templates:
      - key: uri
        values:
          - /get
          - /get?foo=bar
      - key: command
        values:
          - php
          - php%20foo
    stages:
      - input:
          uri: "{{ .uri }}"
          method: POST
          data: "cmd={{ .command }}"
        output:
          log:
            expect_ids: [932160]

The above generates 4 test instances (2 URIs × 2 commands).

Other changes

• spec/v2.4.0/ — new spec directory with regenerated JSON schemas reflecting the updated types
• examples.go — added ExampleTemplateTest, ExampleTemplates, and supporting example values

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch copilot/add-dynamic-tests-feature

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[fzipi]

@copilot apply changes based on this comment: #63 (comment)

[theseion]

I love it ❤️.