examples: add Fastify REST API demo with plugin and route preHandler

[Kareem411]

Goal

Add a runnable Fastify REST API demo in examples/fastify-api/ demonstrating TriCache's official Fastify caching plugin (tricache/http).

Scope

1. Create a minimal Fastify application in examples/fastify-api/.
2. Demonstrate both usage styles supported by createFastifyPlugin:
   • Global plugin registration (fastify.register(createFastifyPlugin(cache, options)))
   • Route-level preHandler hook (fastify.get('/route', { preHandler: plugin.preHandler }))
3. Showcase early short-circuiting in onRequest, response payload capture in onSend, weak ETag calculation, and 304 Not Modified conditional responses.
4. Include a clean README.md with instructions to run locally and test via curl -i.

References

• Implementation: src/http/fastify.ts
• Documentation: Express & Fastify Guide