Nursery: support dynamic provider configuration

[scuba10steve]

Add the ability to dynamically configure nursery providers at runtime. This is needed to support the UI provider management feature (orchard-ui#37).

Requirements:

• Allow multiple providers of the same type (e.g. two SSH provider instances)
• Each provider instance has a UUID for its internal ID
• Each provider instance has a human-readable alias (name)
• Expose CRUD API endpoints for managing provider configurations
• The provider registry should be dynamic — adding/removing providers should not require a server restart
• Define the API contract (swagger/openapi) so orchard-ui can consume it

Depends on: orchard-ui#36 (read provider info)