Configurable default feature values

[reckart]

Is your feature request related to a problem? Please describe. Primitive features always start with a system-defined default value, e.g. the empty string for string fields.

Describe the solution you'd like It would be nice to be able to configure the default value for each feature.

Additional context Original feature request: https://github.com/webanno/webanno/issues/1687#issuecomment-634165512

To do

• [x] String features
• [ ] Boolean features
• [ ] Integer features
• [ ] Float features
• [ ] Concept features