Custom permission models through webhooks

[semekh]

Is your feature request related to a problem? Please describe.
Boundary's uses RBAC-based permission model, and while being good enough for some use cases, it might not meet the requirements of those in need of more fine-grained access control.

Describe the solution you'd like
If Boundary could query an external endpoint like "Does user X have access to perform A on object B?" it would let the organization decide how it's going to authorize actions.

Describe alternatives you've considered
Permissions could be handled separately for Boundary, but not all permission models fit into RBAC. This make handling permissions error-prone and costly.

Explain any additional use-cases
Adopting a Zanzibar-like system to model assets and permissions and managing permissions centrally reduces the operation burden of keeping permissions in-sync significantly. Providing such an extension point makes it easier to adopt such models.

Additional context