Sync Open API specifications with latest additions

[manojradhakrishnan]

The Open API specification needs to be updated to sync with:

• [ ] newly implemented APIs such as /session and /verify #265
• [ ] the addition/ changes in parameters of existing APIs shall also be modified in the specifications
• [ ] payment status fields, description and significance
• [ ] documentation of all possible API errors, explanation and significance
• [ ] documentation of all webhook events with explanation
• [ ] include the health check endpoint (/health) in the spec

[bernard-eugine]

#263