Implement missing $value serialization modifier for GET requests #644

This issue regroups the already reported missing endpoints in #528, #529, #530 and #531 by underlying
mechanism rather than endpoint path. Part of #439. Originally requested in #446.

Core issue here is the missing ValueOnly serialization ($value modifier) for reading Submodels and
SubmodelElements.

Affected endpoints:

• GET /submodels/$value (server/app/interfaces/repository.py: Implement missing /submodels endpoints #528)
• GET /submodels/{submodelIdentifier}/$value (server/app/interfaces/repository.py: Implement missing /submodels/{submodelIdentifier} endpoints #529)
• GET /submodels/{submodelIdentifier}/submodel-elements/$value (server/app/interfaces/repository.py: Implement missing /submodels/{submodelIdentifier}/submodel-elements endpoints #530)
• GET /submodels/{submodelIdentifier}/submodel-elements/{idShortPath}/$value (server/app/interfaces/repository.py: Implement missing /submodels/{submodelIdentifier}/submodel-elements/{idShortPath} endpoints #531)

Scope:

• One shared ValueOnly serializer used by all four routes
• Support the level query parameter (deep/core) on all four routes
• Support limit/cursor pagination on the two collection routes, and the semanticId/idShort
  filters on GET /submodels/$value, as on the existing collection endpoints

extent and writing values via PATCH .../$value are out of scope and tracked separately.

Endpoints available via the AAS superpath (/shells/{aasIdentifier}/submodels/...) are covered
automatically through the existing redirect.