extension: OperationOptions extension

[dobegor]

Design and implement an operation-level OpenAPI extension that would allow i.e. disable a specific operation or configure some settings such as CORS or rate limiting on a path-based level.

The definition would perhaps reside in options package. The code to extract it from openapi3.Operation.ExtensionProps would most likely be in spec package.

Features:

• enable/disable
• cors origins
• rate limit in requests per second ?

Issues to implement in generators would follow:

• Implement OperationOptions extension in Ambassador (enable/disable, cors) #45
• Implement OperationOptions extension in Linkerd (enable/disable) #46
• Implement OperationOptions extension in nginx-ingress (enable/disable, cors) #47