feat(selfservice/strategy/oidc): add spotify provider

[xkisu]

This pull request adds a self-service OIDC provider for Spotify's accounts API and supporting documentation.

While Spotify makes it appear that they support OIDC and they provide a discovery URL, they do not actually support OpenID Connect. Any attempt to use an OIDC client with Spotify will result in errors from Spotify about the openid scope, which makes it incompatible with Kratos' "generic" provider as it understandably injects the openid scope into the list of scopes you configure it with.

This pull request adds a new provider to the OIDC strategy that creates a suitable oauth2 configuration for Spotify's nuances and then uses the Spotify API to retrieve the user's profile and account information.

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security. vulnerability, I
  confirm that I got green light (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

Originally I thought it would be possible to use the generic OIDC provider to interface with Spotify, but I quickly realized that their OpenID Connect implementation was not an actual implementation that was up to spec, and at best was very basic OAuth2.

Due to not being an actual OpenID Connect provider, the Spotify accounts API did not work directly with OIDC's generic provider because of the required openid scope not being supported by Spotify (despite Spotify indicating otherwise) and lacking claims in the response data due to an additional API call being required to retrieve the account information from Spotify.

[aeneasr]

Sweet! As far as I can tell this looks good to go for me :)

[xkisu]

Sweet! As far as I can tell this looks good to go for me :)

Oh awesome! I'd left it as a draft while I was trying to figure out if I needed an issue with a design document for it or not