Add method list_functions

[vcnainala]

For completeness, the API should provide a method list_functions(), which should return a list of FunctionDescriptor objects containing the internal code, name and human-readable description of each supported operation, therefore, making the API self-documenting and callable by UI-based user frontends, which would be able to invoke the list_functions()method and present available options to the user in the UI (as a choice list, tree view, whatever). If compute is called with invalid function codes, the method should fail with an error. If, for some input structures the requested values could not be computed, the corresponding row/columns in the output table should containg NULL’s (a.k.a. null’s or none’s).

Optionally, an extra field (table column) can be added to the output to contain the status message for each result of an operation.

[vcnainala]

Firstly, the microservice is meant to serve front ends and/or workflows via API requests.
OpenAPI JSON already serves all the details about the list of endpoints available and necessarily associated metadata, should anyone want to populate a front-end interface or UI with available options.

Implementing invokable code execution from user input via API could also pose major security concerns that must be addressed.
The only use case I see is to allow users to extend the functionality without having to extend the API endpoints.

[vcnainala]

Closing this in favour of #383