feat: Add Fastify caching plugin adapter #7

🎯 Motivation

TriCache currently provides first-class adapters for Next.js, NestJS, Prisma, Drizzle, and raw HTTP (src/http/).

Adding a native Fastify plugin (tricache/fastify or src/fastify/) would make TriCache seamless for the high-performance Fastify ecosystem.

💡 Proposed Solution

Create a Fastify plugin that:

1. Registers a fastify.cache decorator instance with TriCache CacheService.
2. Supports route-level caching options:

   fastify.get('/users/:id', {
     config: {
       cache: {
         ttl: 60,
         tags: (req) => [`user:${req.params.id}`]
       }
     }
   }, handler);

3. Automatically handles cache hits and responses with x-cache: HIT | MISS | STALE headers.

📂 Starting Points

• Look at src/http/index.ts for reference HTTP cache wrapping.
• Look at src/nestjs/index.ts for decorator patterns.
• Vitest tests should be added to tests/fastify.test.ts.

Feel free to open a PR or comment if you would like to work on this!