Introduce parameter for adding additional scopes

[Shocktrooper]

As I was trying to configure this plugin I noticed that only the openid scope is sent which is required in OIDC flows. For OIDC providers like okta preffered_username only comes with the profile scope. This has prevented us from using our orgs IDP with this plugin

$oidc->addScope('my_scope'); looks like it needs to be added to the plugin
Reference

• https://developer.okta.com/docs/api/openapi/okta-oauth/guides/overview/#scope-values
• https://github.com/jumbojett/OpenID-Connect-PHP#example-4-request-client-credentials-token

[Shocktrooper]

Probably should make this configurable incase preferred_username is in something other than the profile scope for other IDP's