DEV: Unify params access in services

Currently, there are two ways (kind of) for accessing `params` inside a
service:
- when there is no contract or it hasn’t been reached yet, `params` is
  just the hash that was provided to the service. To access a key, you
  have to use the bracket notation `params[:my_key]`.
- when there is a contract and it has been executed successfully,
  `params` now references the contract and the attributes are accessible
  using methods (`params.my_key`).

This patch unifies how `params` exposes its attributes. Now, even if
there is no contract at all in a service, `params` will expose its
attributes through methods, that way things are more consistent.

This patch also makes sure there is always a `params` object available
even when no `params` key is provided to the service (this allows a
contract to fail because its attributes are blank instead of having the
service raising an error because it doesn’t find `params` in its context).

lib/service/base.rb

@@ -441,8 +441,12 @@ module Service
     def initialize(initial_context = {})
       @context =
         Context.build(
-          initial_context.merge(__steps__: self.class.steps, __service_class__: self.class),
+          initial_context
+            .compact
+            .reverse_merge(params: {})
+            .merge(__steps__: self.class.steps, __service_class__: self.class),
         )
+      initialize_params
     end
 
     # @!visibility private
@@ -463,5 +467,21 @@ module Service
       context["result.step.#{step_name}"].fail(error: message)
       context.fail!
     end
+
+    private
+
+    def initialize_params
+      klass =
+        Data.define(*context[:params].keys) do
+          alias to_hash to_h
+
+          delegate :slice, :merge, to: :to_h
+
+          def method_missing(*)
+            nil
+          end
+        end
+      context[:params] = klass.new(*context[:params].values)
+    end
   end
 end