Add model caching for LabelScorer creation

[SimBe195]

For the Encoder class we have a EncoderModelCache which can be used to cache associated models (e.g. ONNX models for the OnnxEncoder) to avoid loading them multiple times when creating multiple instances. This PR adds a similar mechanism for LabelScorer creation. Since some LabelScorers use multiple models (e.g. StatefulOnnxLabelScorer with the initializer-, updater- and scorer-model), the cache is keyed by the corresponding config selection. The cache class has been unified with the EncoderModelCache into a common class living in src/Nn/LabelScorer/ModelCache.hh.