webhooks: expand auth configuration

The purpose of this change is to enable internal webhooks to be
configured in GQL webhook tables. A webhook subscription now includes
the auth method field which is appropriate, which is limited to either
OAUTH2 or INTERNAL. In the former case, the previous set of fields will
be valid, and in the latter case, the NodeID field will be valid. This
will allow us to register webhook subscriptions for internal use.

webhooks: panic on nested webhook auth case

This is not strictly speaking necessary since we have a panic which
immediately follows the switch, but it is cleaner.

webhooks: initial prototype for GQL-native webhooks