Badgr integration updates (#27181)

* Badgr integration fix

Badges are no longer created on the Badgr side with a given
'slug'. Instead, a slug(v1) or an entityId(v2) will be generated
for each badge on the Badgr side and we will need to use that
value to check if a certain badge matching a BadgeClass on our side
exists and/or to create assertions for it.

This commit introduces a new field to the badgeclass:
'badgr_server_slug' by cherry-picking the following commit from
3309aab2a2eb00d28c5ca3d3145c8dddb15e6159
- TTK-18543: fix Badgr Server
connection (https://github.com/teltek/edx-platform/pull/46

)

This commit also modifies the cherry-picked commit by making the newly
added field optional since the BadgeClass is not neccessarily always used
with the Badgr backend.

Co-authored-by: mrey <mrey@teltek.es>

* Implement OAuth2 tokens flow for BadgrBackend

* Use Badgr v2 API

Co-authored-by: mrey <mrey@teltek.es>