feat(gatsby-plugin-fastify): allow adding custom error/404 handlers

[moonmeister]

Is your feature request related to a problem? Please describe.

#105

also people might want to implement some kind of log tracking. figure out how to best do these.

Describe the solution you'd like

not really sure. could be a function where users can do anything. need to understand pino better and maybe errors and 404 should be handled differently.

[moonmeister]

I believe this is somewhat/now handled via #321 and https://www.fastify.io/docs/latest/Reference/Server/#frameworkerrors. We can revisit in the future if needed.