make document loaders a standalone crate in repo

[cs50victor]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when I want to use document loaders for a simple task but I have to import the entire langchain crate

Describe the solution you'd like
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I would love to import document_loaders from the langchain repo directly in my toml file.

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[prabirshrestha]

Is feature flag not enough? We can add more if needed. Do you mean each document loader becomes its own crate or just have all document loaders in one crate behind feature flags is enough?

[cs50victor]

ideally i would love if i could import all document loaders in one crate

document_loaders = { git = "https://github.com/Abraxas-365/langchain-rust" }