Support Facebook as OIDC Provider

[qvamatel]

Is your feature request related to a problem? Please describe.

Because there is no facebook provider, I attempted to integrate with Facebook using the generic oidc provider like so:

...
oidc:
  enabled: true
  config:
    providers:
      - id: facebook
        provider: generic
        mapper_url: file:///etc/config/kratos/oidc.facebook.jsonnet
        client_id: ID
        client_secret: SECRET
        scope:
          - openid
          - email
        issuer_url: https://facebook.com
        auth_url: https://facebook.com/dialog/oauth
        token_url: https://graph.facebook.com/oauth/access_token
...

with oidc.facebook.jsonnet containing

local claims = std.extVar('claims');

{
  identity: {
    traits: {
      name: claims.name,
      [if "email" in claims then "email" else null]: claims.email,
    },
  },
}

Because facebook uses its own solution and does not fully support to OpenID Connect Discovery, so kratos runs on error, like:

https://community.ory.sh/t/kratos-facebook-sso/1748

Describe the solution you'd like

Requires a facebook provider similar to the github provider: #519

Describe alternatives you've considered

I'll open a draft pull request shortly with what I was able to get working.

I worked with the following documentations:

https://developers.facebook.com/docs/facebook-login/manually-build-a-login-flow

https://developers.facebook.com/docs/graph-api/reference/user