refactor: better engine mounting

[adrianthedev]

Description

This PR refactors the way the Avo engine is mounted. Same for other Avo plugins.
This should significantly improve the way we recommend users to mount Avo when they are using the scope option.
I guess we could even remove the self.mount_avo_engines configuration option at some point.

# Other engines use this API to self-mount themselves
Avo.plugin_manager.mount_engine Avo::Collaborate::Engine, at: "/collaborate"

# like so...
module Avo
  module Collaborate
    class Engine < ::Rails::Engine
      isolate_namespace Avo::Collaborate

      initializer "avo-collaborate.init" do
        ActiveSupport.on_load(:avo_boot) do
          Avo.plugin_manager.register :avo_collaborate

          Avo.plugin_manager.mount_engine Avo::Collaborate::Engine, at: "/collaborate"
        end
      end
    end
  end
end

TODO

• make mount_avo take a at: argument
• cleanup
• tests
• update all other plugins to self-mount themselves
• docs

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

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[github-actions]

[rubocop] _{reported by reviewdog 🐶}
[Corrected] Layout/SpaceInsideHashLiteralBraces: Space inside { detected.

[github-actions]

[rubocop] _{reported by reviewdog 🐶}
[Corrected] Layout/SpaceInsideHashLiteralBraces: Space inside } detected.