[ADD] portal: manage api keys in portal

Add the possibility for portal users to manage API keys
in the frontend portal interface.

- Allow for portal users to manage their API keys if the system parameter
  `portal.allow_api_keys` is set.
  A settting in `res.config.settings` is added in order
  to enable or disable the feature (to add/remove the system parameter)
  through a compute field,
  to avoid adding a new database column in standard
  (as this revision targets stable 15.0).
  The setting appear only in debug mode.

- Display the API keys in the portal in debug mode only
  (as in the back-end for regular users, in their profile)

- The flow in the frontend is a mimic of the flow from the backend,
  the wording and the look and feel is from the back-end.

closes odoo/odoo#86915

Signed-off-by: Denis Ledoux (dle) <dle@odoo.com>