feat: enhance .file for loading resources from diverse sources

[sigoden]

After this PR is merged, .file supports the new syntax .file <loader>:<resource>, and .rag also supports the new type of resource <loader>:<resource> when adding documents.

document_loaders:
  jina: 'curl -fsSL https://r.jina.ai/$1 -H "Authorization: Bearer <jina_api_key>"'  # jina.ai is free
  git: >
    sh -c "yek $1 --json | jq '[.[] | { path: .filename, contents: .content }]'"

.file jina:https://huggingface.co/deepseek-ai/DeepSeek-R1 -- 
     What is the difference between DeepSeek-R1-Zero and DeepSeek-R1

> .rag
...
? Add documents: jina:https://huggingface.co/deepseek-ai/DeepSeek-R1

close #1123