Add support for BERT embedding models (#5423)

Add support for BERT embedding models (#5423)

* BERT model graph construction (build_bert)
* WordPiece tokenizer (llm_tokenize_wpm)
* Add flag for non-causal attention models
* Allow for models that only output embeddings
* Support conversion of BERT models to GGUF
* Based on prior work by @xyzhang626 and @skeskinen

---------

Co-authored-by: Jared Van Bortel <redacted>
Co-authored-by: Jared Van Bortel <redacted>
Co-authored-by: Georgi Gerganov <redacted>