Add an example of Fastify and `@typescript-eslint/no-floating-promises`

Following https://github.com/fastify/fastify/issues/5498 -> https://github.com/fastify/fastify/pull/5506, I don't know that there's a single reproduction online showing using [`allowForKnownSafePromises`](https://typescript-eslint.io/rules/no-floating-promises/#allowforknownsafepromises). https://fastify.dev/docs/latest/Reference/TypeScript shows how to get started.