Webhooks API - Allow defining OAuth2 scopes for webhook callback URL

Description

This change adds an optional scope property to the callbackAuthentication object of type oauth2 in the POST /webhooks-api/v201907/subscriptions request body params.
It allows customers to define scopes that will be passed to the token service used by SmartRecruiters to obtain an access token for the webhook callback URL.

Impact

This is a non-breaking change, as it only adds a parameter to the request of a single endpoint.
Clients do not need to update or modify anything in their existing codebase.

References