Config schema export

[brettviren]

This is the start of an idea to better handle configuration. It's in the same vein as the moribund #105 but with more of an adiabatic plan. To start with, all this does is dump a JSON representation of component metadata:

wire-cell -a ConfigSchema \
    -p WireCellApps -p WireCellGen -p WireCellAux -p WireCellSigProc -p WireCellImg \
    | json_pp > schema.json

The JSON representation includes:

• The WCT "type" name, C++ class name and names of all the interfaces
• If the component is an IConfigurable then the schema describing the object returned by default_configuration() is generated, with the default values included.
• If the component is an INode then the category, signature and data types passed by the input/output ports are included.

Note: since not all IConfigurable implementations return a full object from default_configuration() the dumped schema will not be complete.

Long term, the idea is that this dump can turn around and become human-developed and used to generate c++ config structs a'la #105. Until then there are plans to exploit the dump to generate some added value: documentation, Jsonnet helpers, graph validation.