server: Implement the read-only $value routes

[thammel]

All $value routes previously responded with 501 Not Implemented.

The specification leaves some points open, which are resolved as follows:

• Property values are mapped to the JSON type of their valueType, so booleans and numbers are not quoted. Values without a JSON literal are returned as their XSD representation. xs:integer is unbounded in both Python and JSON and is therefore passed on as it is.
• Elements without a value are omitted from Submodels, collections and Entity statements, but kept as null within a SubmodelElementList, as omitting them would shift the indices of the remaining elements.
• Operations and Capabilities have no ValueOnly representation. They are skipped in containers and result in 400 Bad Request when requested directly, which is how the $metadata route handles them.
• level=core returns the direct children of the requested resource and their nested containers as empty, matching the examples of the specification.
• The entries of GET /submodels/{id}/submodel-elements/$value are mapped by idShort, as a bare SubmodelElementValue would lose the association between a value and its element. Submodels are returned as-is, since their entries are already named.
• The languages of a MultiLanguageProperty are sorted, to keep the order of the response deterministic.

The ValueOnly serialization is only defined for JSON, so the routes respond with 406 Not Acceptable if XML is requested. extent remains unimplemented, as on all other routes. PATCH …/$value, the $path routes and the operation invocation routes are not part of this change.

Fixes #446