Use Conn#config instead of Conn#bag to store values needed by the extensions

[waiting-for-dev]

This stops polluting Conn#bag hash keys. Conn#bag is intended for application level values, while Conn#config should contain configuration values that extensions may need.

This breaks :container, :dry_view and :dry_schema extensions, which were currently reading :container, :view_context and :param_sanitization_handler keys from Conn#bag. Besides, :view_context should now be a lambda accepting the Conn instance at the moment Conn#view is executed.

Specialized plugs for those three extensions which were used just for the purpose of adding needed keys to #bag have been removed. Instead, a WebPipe::Plug::Config now exists which can set in a single shot all settings provided:

plug :config, WebPipe::Plug::Config[
  container: MyContainer,
  view_context: (conn)-> { name: conn.fetch(:name) },
  param_sanitization_handler: (conn, result) { ... }
]