⬆️ [Upgrade 0.29] Publik module

[luciegrau]

Description of need

Make the module https://github.com/OpenSourcePolitics/omniauth-publik compatible with version 0.29 of Decidim in order to add it to our Decidim-app 3.1.

[Stef-Rousset]

PR => #8

There is no way to test the PR in local env. I just added the module to decidim-app rc-3.0.0 and run the app in local env, just to check that there was nothing preventing the app to run.