Draft: DBLab API redesign (4.0)

WIP – NOT YET IMPLEMENTED

Proposed API redesign – unified simplified (most decisions inspired by Stripe API):

• plural in most endpoints (GET /clones, POST /clones, GET /clones/{id}, PATCH /clones/{id}, etc.)
• flatten parameters – no nesting
• adjusted error codes and messages
• adjusted return structures
• GET /status doesn't provide clone and snapshot lists anymore – since we now have GET /clones, GET /snapshots

API docs:

• DBLab 4.0-alpha.5 API, before redesign: https://dblab.readme.io/ (can be tested at https://branching.aws.postgres.ai:446/, demo-token)
• As proposed in this MR: https://dblab.readme.io/v4.0.0-redesigned/ (not yet implemented)