gguf-py, convert-hf : model conversion support for T5 and FLAN-T5 model variants (#5763)

* gguf-py : add T5 model architecture

* gguf-py : add separate tensors for encoder and decoder

* gguf-py : add new model header parameters: decoder_start_token_id, attention.relative_buckets_count, tokenizer.ggml.remove_extra_whitespaces, tokenizer.ggml.precompiled_charsmap

* convert-hf : add model conversion support for T5ForConditionalGeneration and T5WithLMHeadModel

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Co-authored-by: Stanisław Szymczyk <sszymczy@gmail.com>