[Alerting] Add Generics support to Alerting

[gmmorris]

Work on #64145 has revealed a potential issue in our framework.

We currently use any instead of unknown throughout all types which interact with solutions - this has the potential of breaking the typing in solutions without them knowing and makes it harder to interact with our framework as manual conversions are forced on anyone using the framework and due to any's permissive nature it can actually cause solutions' code to typecheck incorrectly.

Changing the any to unknown will prevent the permissive typechecking issues, but will still require manual conversions and will require fixing all solutions already integrated.
It's worth investigating the use of generics as that will allow us to default to any, which means we won't break existing usage`, but that will offer solutions the option of using generics to get the exact type they expect to get and not need to typecheck or convert themselves.

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)

[gmmorris]

It's worth noting that in order for this to work well we probably want to add generics in TaskManager as well so that you can specify the type there and remove the current use of any too.