Replace decorateRequest by onRequest hook

[andreffvalente]

Resolve deprecation warning triggered by Fastify when decorating the Request with a reference type.

DeprecationWarning: You are decorating Request/Reply with a reference type. This reference is shared amongst all requests. Use onRequest hook instead. Property: oas

This replaces the fastify.decorateRequest() with the fastify.addHook('onRequest', ...).