feat: fastify v4 support

[Eomm]

Fixes #10

This PR adds a lightweight Fastify v4 support:

1. The NodeJS versions are the one listed in the LTS policy
2. So fastify v4 is not tested against NodeJS 14, 16, 18
3. The fastify-plugin has been removed so we don't have any blockers based on this module version
4. The @opentelemetry/* deps were always the same used for all the fastify's version
5. We may add support for fastify v3, but this would be just a jump of faith because:

• The supported hooks are a subset of the current listed here
• The @opentelemetry/* was testing fastify v4 only
  • Ref: feat(instrumentation-fastify): Enable Fastify v5 open-telemetry/opentelemetry-js-contrib#2460
• The @opentelemetry/* was testing old Node.js versions
  • ["14", "16", "18", "20", "22"]

So I think that:

• adding support for v4 is something needed by the community
• adding support for v3 is something that someone should sponsor if needed

I think this because of this output too: https://craigory.dev/npm-burst/?package=fastify
The 61.6% of our users are still using fastify v4.

[mcollina]

It should be tested against Node v18 - it's still there.

[Eomm]

It should be tested against Node v18 - it's still there.

Done - I just adapted a bit tests:

• removed t.plan because it is not there
• find/replace t.assert to assert

Checked that the tests fail accordingly if the assertions are not good of course:

[mcollina]

lgtm