Facebook auth adapter appSecret cannot be set per app ID

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest version of Parse Server.

Issue Description

The Facebook auth adapter takes two config options:

• appIds: type of array of strings
• appSecret: type of string

While it's possible to set multiple appIds, it's only possible to set one appSecret. Every Facebook app has its own secret, which means that appSecret can only be used if appIds only has a single array element, i.e. Parse Server is configured to support only one Facebook app.

Steps to reproduce

n/a

Actual Outcome

It's not possible to set a secret per Facebook app ID.

Expected Outcome

It should be possible to set a secret per Facebook app ID, e.g. with the following config:

auth: {
  facebook: {
    apps: [
      {
        id: 'id1',
        secret: 'secret1'
      },
      {
        id: 'id2',
        secret: 'secret2'
      }
    ]
  }
}

As this is a breaking change, Parse Server should temporarily accept the apps option in addition to the current config options, which should be deprecated.

Environment

Server

• Parse Server version: 5.3.0-alpha.26

References

• https://developers.facebook.com/docs/facebook-login/security/#appsecret

Use the Facebook auth adapter's current appIds/appSecret handling as the entry point. Compare the proposed apps structure with existing configuration compatibility requirements and identify coverage for both new per-app secrets and deprecated options. Done means multiple Facebook app IDs can each authenticate with their own secret while existing configuration remains temporarily accepted.