Add configurable options

[erral]

We have been working to use EU Login with this plugin because it implements OpenID connect.

Nevertheless we have found that we needed to do some changes in the implementation, such as:

• Add a configurable option to select which scopes to use
• Add an option to use PKCE
• We found that EU Login has some strange implementation, where the email_verified and phone_number_verified boolean values are sent as string values ('true'), so the automatic validation that oidc library does fails with EU Login. So we have added an option to use a modified version of the validation that accepts 'true' values as real JSON true and thus python True.
• We have also found that EU Login does not have the userinfo endpoint, so we now check if the endpoint exists before querying it. If the endpoint is not defined, we extract the userinfo from the id_token, with is already decoded in the previous step (obtaining the access_token).

We want also to work on the came_from/next_url thing, but had no time yet. Will check it after holidays :)

[mamico]

LGTM