create_key () creates a key that can be used to store and retrieve values. The type needs to be indicated explicitly.
Example
let string_key: string = Sihl.Core.Ctx.create_key () in
let int_key: int = Sihl.Core.Ctx.create_key () in
let foo_key: Foo.t = Sihl.Core.Ctx.create_key ()
Map
type t
The service request context is a heterogeneous map that can store values of different types. It is typically used to pass values to services that are either 1) only valid in the context of a service request or 2) whose types are hidden so that different service implementations of the same interface can take different values.