chore: Add Instant Clusters to OpenAPI spec (#168)

* chore: Add Instant Clusters to OpenAPI spec

* move list to /gpu_cluster path

* move shared_volumes to shared_volume path

* Update openapi.yaml

* rename secret per request from Yogish

* Update url references

* fix

* fix required fields

* move update to url path put

* add path parameter

Author: blainekasten