OAuth2: add flag to hash token request parameters into state

[creativecoder]

We're seeing more services implement strict url matching with the redirect URI during an OAuth 2.0 authentication flow.

• This is already implemented for Instagram basic display API
• Mailchimp recently announced they would be enforcing this starting in Nov 2020.

Rather than doing this ad-hoc for each service, it would be more efficient to be able to opt in to this for any Oauth2 connection.

Idea from @pablinos

As using only the state parameter is the OAuth standard, I'm wondering if we should add the filter to Keyring_Service_OAuth2 if a flag is set ($this->pack_state = true in the constructor perhaps?), and then slowly convert the services.