Add a `hint` option for `before_send`

[greenhat616]

As JavaScript SDK did, they provide a params called hint, which we can modify some scoped content, such as attachments.

In current state, we had no way to configure the attachments only when level is Fatal

• https://docs.sentry.io/platforms/javascript/enriching-events/attachments/#add-or-modify-attachments-before-sending