What kind of change does this PR introduce?

• Adds the slack_oidc provider, which supports the slack v2 API
• Adds the linkedin_oidc provider
• Adds 3 new fields to the admin create user method - role, password_hash and id, which allows one to specify a role, password hash or a custom id when creating a user through the admin method

What is the current behavior?

Please link any relevant issues here.

What is the new behavior?

Feel free to include screenshots if it includes visual changes.

Additional context

Add any other context or screenshots.