Add multi-file write support

[0div]

Description

Create separate multipart.Part method to get file name with full path with directories.

Tests

See the tests in e2b-dev/E2B#451

[linear]

E2B-954 Add a way to upload several files to a sandbox at once

[0div]

@ValentaTomas per discussion in e2b-dev/E2B#451, I overloaded the multipart.Part.FileName() method so that it's possible to get the full path with directories. It's unsafe for regular web apps, but in our sandboxes it's ok, correct? I adapted the js-sdk to include nested paths and it works.

[ValentaTomas]

Yes, I think it should be okay for us because we are giving users access to the whole sandbox, so this is not a security risk.

[ValentaTomas]

Is it tested for the old file uploads via SDK when passing relative and absolute path and uploads via URL (with the path being part of the multipart)?
I just want to make sure it is compatible with the current usage.

[0div]

Is it tested for the old file uploads via SDK when passing relative and absolute path and uploads via URL (with the path being part of the multipart)? I just want to make sure it is compatible with the current usage.

New write tests have passed as follow: https://github.com/e2b-dev/E2B/pull/503/files#diff-7a70570a2ea87cb7d3e355b0aa6f011a4bec4d1733f40bf94b05e4024f232b01