Support for customized tenant provider

[bschrag]

Hello,

I am working on a project that needs to allow for organizations to configure their own google oauth integration for SSO. Looking through documentation and code, it didn't seem that this is yet supported out of the box. I was able to achieve it with a simple module and including it into OmniAuth::Strategies::GoogeOauth2 in my devise.rb initializer prior to loading it:

# config/initializers/devise.rb
module GoogleOmniAuthProviderCustomization
  def self.included(klass)
    klass.option :tenant_provider, nil
  end

  def client
    provider = if options.tenant_provider.class.is_a?(Class)
      options.tenant_provider.new(self)
    else
      options
    end
    options.client_id = provider.client_id
    options.client_secret = provider.client_secret

    super
  end
end

OmniAuth::Strategies::GoogleOauth2.include(GoogleOmniAuthProviderCustomization)

Devise.setup do |config|
  ...

  config.omniauth :google_oatuh2, tenant_provider: MyCustomGoogleProvider

  ...
end

If this is something that you feel would be of value I'm happy to submit a PR to extend the current functionality. Thanks for your time.