Allow setting requestContext outside of a fastify route

[Floriferous]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

We're listening to webhooks on our fastify backend with GCP's pubsub, they offer a library that handles the routing of those webhooks, and is therefore not done via a fastify route. Some of these requests can be authenticated with the payload of the webook.

We're using this package to store the auth object and easily be able to retrieve it later.

It'd be great if it was possible to set the request context outside of a fastify handler, and let the rest of our application code still work as if it was called with a proper request context.

Right now, if I try to use requestContext.set from this webhook, it does not allow me to retrieve this data later on.

Motivation

No response

Example

No response

[kibertoad]

PR would be welcome

[Floriferous]

Similarly, when running tests, you'd want to be able to set the context before your test, and then have the rest of your code behave as intended.