Add convert_text_to_mds to CLI

[KuuCi]

This PR allows users to call llmfoundry convert_text_to_mds {ARGS} while maintaining correctness with existing convert_text_to_mds script. The motivation is for DLE where we want to make the CLI much more intuitive in the docker images

[KuuCi]

test-data-txt-LJgtNP runs:
llmfoundry convert_text_to_mds \ --output_folder my-copy-shakespeare \ --input_folder shakespeare \ --concat_tokens 2048 --tokenizer EleutherAI/gpt-neox-20b \ --compression zstd

test-data-txt-du60oE runs:
python data_prep/convert_text_to_mds.py \ --output_folder my-copy-shakespeare \ --input_folder shakespeare \ --concat_tokens 2048 --tokenizer EleutherAI/gpt-neox-20b \ --compression zstd image: mosaicml/llm-foundry:2.3.1_cu121-latest

[KuuCi]

txt-cli-GVwCPT Runs:
llmfoundry convert_text_to_mds \ --output-folder my-copy-shakespeare \ --input-folder shakespeare \ --concat-tokens 2048 --tokenizer EleutherAI/gpt-neox-20b \ --compression zstd

txt-orig-Zagm3I Runs:
python convert_text_to_mds.py \ --output_folder my-copy-shakespeare \ --input_folder shakespeare \ --concat_tokens 2048 --tokenizer EleutherAI/gpt-neox-20b \ --compression zstd

[KuuCi]

txt-cli-Y2HXxb runs:
llmfoundry data_prep convert_text_to_mds \ --output-folder my-copy-shakespeare \ --input-folder shakespeare \ --concat-tokens 2048 --tokenizer EleutherAI/gpt-neox-20b \ --compression zstd

txt-orig-tFXNED runs:
python convert_text_to_mds.py \ --output_folder my-copy-shakespeare \ --input_folder shakespeare \ --concat_tokens 2048 --tokenizer EleutherAI/gpt-neox-20b \ --compression zstd