Reduce duplicated API dispatch logic around Ramose

[arcangelo7]

I noticed that oc_api.py seems to repeat part of the logic that Ramose already provides.

The overlap is mainly in the REST API dispatch layer:

• selecting the right APIManager
• matching a request to a Ramose operation
• calling Operation.exec()
• translating status, body, content type, and extra headers into an HTTP response
• serving API documentation and OpenAPI output
• handling some API errors that Ramose already returns