Add support of modern decoders as a backbone

[Ingvarstep]

• Add new entity representation schema for better support of other models;
• Adapt GLiNER for modern LLMs as backbones;
• Add support of T5 encoder as a backbone;

[Ingvarstep]

Here is fine-tuned GLiNER model with Tiny Llama as a backbone: https://huggingface.co/knowledgator/gliner-llama-1B-v1.0