Provide a way to know which features are supported

[duglin]

E.g. model, specversion, inline, filter, export...

Should probably be part of the model, but read-only in API view since support requires code

[duglin]

Proposal:

• add a "features" map to the model
• key = feature name
• value = any
  • false not supported
  • true is supported
  • any other value (including {}) mean it is supported
  • feature can define the type/shape of the value to allow for fine-grained config
  • must generate an error if a type is invalid type/format/values....
• during an update, excluding the feature will delete it and reset it to the impl's default value
• impls SHOULD expose all features even if the values are the spec defined values
• a missing feature from the list MUST mean it has the spec default value