Introduce default permissions

[peterschmidt85]

At the moment we don't have granular permission. However, we want to be able to configure the following permissions defaults via environment variables:

• DSTACK_USER_CAN_CREATE_PROJECTS – Whether a user is allowed to create new projects (True by default)
• DSTACK_USER_CAN_CREATE_GATEWAYS – Whether a user (not a project or global admin) is allowed to create gateways (True by default)
• DSTACK_USER_CAN_CREATE_ON_PREM_FLEETS – Whether a user (not a project or global admin) is allowed to create on-prem fleets (True by default)

[olgenn]

@peterschmidt85 Maybe, do we need a permission: DSTACK_USER_CAN_DELETE_PROJECTS ?

[peterschmidt85]

@olgenn If a user can create a project, then it can delete that project as an owner