Automatic error handler setup for Fastify v5

[andreiborza]

Description

Fastify v5 introduced diagnostics channels which we could use for automatic setup of the error and request handlers instead of users having to run setupFastifyErrorHandler manually.

While we cannot yet get rid of the handler for <v5, we could simplify the setup for users on v5.

See tracing:fastify.request.handler:start and tracing:fastify.request.handler:error.

[mydea]

We should just make sure that nothing breaks if people on fastify 5 also add the handler manually, to avoid confusion there! But def. a good change to do 🥳