Make prompt configurable for oidc offline_access

[commixon]

For OIDC offline_access, the prompt used to be hardcoded to consent:

opts = append(opts, oauth2.AccessTypeOffline, oauth2.SetAuthURLParam("prompt", "consent"))

However, in some OIDC provides this value is not supported (e.g. OneLogin: https://developers.onelogin.com/openid-connect/api/id-token) where it states:

Optional. If used must be set to one of the following:

• login - The user will be prompted with a login dialog.
• none - The user will not be prompted with a login dialog. If they do not have a current session a login_required error will be returned.

This adds the promptType param, set by default to consent but with the ability to explicitly set it in the config and override it.

[JoelSpeed]

LGTM, thanks!