feat: add download resume

[namchuai]

Describe Your Changes

• Add download resume functionality.

Screenshots

1. If cortex found that the file haven't fully downloaded, will ask if user want to continue (resume)

2. If file is already downloaded, will ask if user want to redownload or ignore

Fixes Issues

• feat: Cache model and engine downloads so users can restart download #777

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed

[dan-menlo]

Lgtm, will let @vansangpfiev @nguyenhoangthuan99 verify for code quality

[dan-menlo]

@namchuai Quick check: how will Jan get the download progress from Cortex, in order to display the download progress indicator?

[namchuai]

@namchuai Quick check: how will Jan get the download progress from Cortex, in order to display the download progress indicator?

We haven't decided yet. I think we have the couple of choices:

1. Via API server sent event API and client (e.g, Jan) can connect to and listen to event from cortex (download event, hardware consuming event, model starting/ stopping event, etc.)
2. Via socket. (or any other IPC mechanism)
   cc @vansangpfiev @nguyenhoangthuan99 if you have any suggestion.

IMO, 1 is faster to implement and relatively easy for 3rd party app to adopt. However, I haven't played with Drogon's stream API and SSE though.