Add Initial Scalar Support

[bnasslahsen]

Add first-class Scalar support

Is your feature request related to a problem? Please describe.
Spring Boot services need a lightweight, built-in API reference UI and testing console.
Today, each service wires this manually, which is repetitive and inconsistent.
Scalar is an open-source API platform that provides tools for documenting, testing, and discovering APIs. It ships as a WebJar with Spring Boot auto-configuration to expose an API reference endpoint, but our project doesn’t yet integrate it out of the box.

Describe the solution you'd like
Provide automatic Scalar integration in Spring Boot–based projects via our starter(s):

• Add two modules to cover both stacks:
  • springdoc-openapi-starter-webmvc-scalar for Spring WebMVC
  • springdoc-openapi-starter-webflux-scalar for Spring WebFlux
• Auto-configure Scalar’s WebJar and register the API reference endpoint.
• Default settings:
  • Enabled by default: scalar.enabled=true
  • Default path: scalar.path=/scalar
• Make behavior fully configurable:
  • scalar.enabled (boolean) to turn the feature on/off
  • scalar.path (String) to customize the mounting path
• Ensure the integration is conditional on the respective web stack being present.

Additional context

• Keep the setup zero-config for most apps, but allow easy override via application.yml.
• Ensure the endpoint respects the application’s context path and works in both MVC and WebFlux.
• Document how to disable the feature per service (scalar.enabled=false) if needed.

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Example configuration

scalar:
  enabled: true
  path: /scalar