Enable Warmup function could be specified outside of the node module.

[marcfielding1]

As an organisation, I NEED to be able to write my own warmup function
SO THAT I can write more complex business logic and attach other event sources.

I think this would simply be a pointer in the yaml that packages a function external to the serverless-plugin-warmup.

The reason this could be useful is shown in my comment, specifically the bit about SNS sources.

[juanjoDiaz]

Just elaborating a bit on this.

I'd suggest that we configure the warmup lambda like any other lambda: handler, trigger, etc.
And have defaults for those.