feat: add shared generator worker module for Gradle and Maven plugins

[vpelikh]

Summary

Adds shared single forked-JVM worker (springdoc-openapi-generator-worker) that boots the application context, lets springdoc-openapi build the spec, writes it to disk, and shuts the context down.
Boots a reactive context with a no-op ReactiveWebServerFactory (fully serverless) or a servlet context on an ephemeral port, writes the spec, then exits.

Changes

New module

• springdoc-openapi-generator-worker/ — shared forked-JVM worker used by both plugins. Detects the target app's stack from its classpath and dispatches to a WebFlux or WebMvc generator.

Design notes

• No per-stack config on the plugin side. The worker infers WebFlux vs WebMvc from the target app's classpath, so a single task/java handles both stacks.
• No fixed port is used. WebFlux never binds a port; WebMvc uses an ephemeral port (0) and shuts down immediately after writing the spec.
• WebFlux is genuinely serverless (no-op ReactiveWebServerFactory); WebMvc starts a real, short-lived container because the servlet model requires a ServletContext.
• The plugin intentionally does not force a springdoc stack API onto the fork classpath; the target app's runtime classpath already provides the stack it needs. This keeps the fork classpath identical in shape to the Maven Mojo.

Follow-ups

• Reuse this module in Gradle and Maven plugins

Part of #3367

[vpelikh]

Hey @bnasslahsen, check this one, please.