Create Fastify integration

[nyeong]

Summary

Implement Fastify intergration.

Related Issue

Reference the related issue(s) by number, e.g.:

• closes Fastify integration #151

Changes

List the specific modifications made in this PR.
Focus on what was changed without going into detail about impact.

• Add packages/fastify
• Add examples/fastify

Benefits

Users now can integrate Fedify into Fastify like:

const fastify = Fastify({ logger: true });
const federation = createFederation<void>({
  kv: new MemoryKvStore(),
});
await fastify.register(fedifyPlugin, {
  federation,
  contextDataFactory: () => undefined,
});

Checklist

• Did you add a changelog entry to the CHANGES.md?
• Did you write some relevant docs about this change (if it's a new feature)?
• Did you write a regression test to reproduce the bug (if it's a bug fix)?
• Did you write some tests for this change (if it's a new feature)?
• Did you run deno task test-all on your machine?

Additional Notes

Remain problems

• Tests for packages/fastify fail on Bun due to timeout errors

[nyeong]

Unless it's crucial, I'm removing examples/fedify from deno.json to skip linting and testing for the example.

Currently, the inclusion of examples in the workspace is inconsistent; some are included, and some are not.