functionality to invalidate cache on a specific key

[fredrikj31]

We need a function to be able to invalidate a cached value.

This can be used, so we can invalidate when we for example updates a new value, then can we invalidate the cache in the database.

We can do one of two things:

1. Populate the fastify instance to have a .clearCache(key) function, which clears the key in the database
2. Have an option on POST/PATCH endpoint that when onSend clears the specific key, which is specified on the endpoint.

[gdgunnars]

Going with option 1 is what I think we should go with.

Making sure the call is always a success (I don't think we should care if the cache existed or not)