feat: allow customizing the namespace for path helpers

[ElMassimo]

Description 📖

This pull request is an attempt to address some of the shortcomings of grouping routes by controllers in apps that have more complex routing:

• gem does not distinguish between nested routes with different params #42
• Incorrect method generated for routes with same URL #48

It adds support for a namespace option in export:

resources :video_clips, export: {namespace: :videos} do
  get :download, on: :member
end

which would then generate a VideosApi.ts file instead of VideoClipsApi.ts.

As a result it's now possible to create custom groupings of path helpers, providing a flexible and intuitive escape hatch for routes that have complex nesting or where the usage in the frontend benefits from a different grouping.

Background 📜

In setups like the following, users can now define a separate file for helpers:

resources :sections, path: "s", param: :slug do
  resources :discussions
end

resources :custom_sections, path: "cs", param: :slug, export: {namespace: "custom_section_discussions"} do
  resources :discussions
end

which will generate another file (CustomSectionDiscussionsApi.ts), containing the second set of paths.

In addition, this pull request also prevents different routes that target the same action from being omitted, in which case it will try to disambiguate additional routes by using their fully qualified name or verb.

FYI @reinvanimschoot @Shaglock