For some reason curl-impersonate (curl_cffi project) does not set the certificate-authority bundle for proxies, which to be fair is for some reason seperated into two curl-options.
Doing this change as well as removing the https->http scheme enforcement on proxies, fixes HTTPS proxies on the curl-impersonate downloaders. I also simplified the seperate http and https proxy definitions to the `all` definition which was not originally supported but does seem to be supported as of v0.6.2.
I tested this on NordVPN proxies which are explicitly HTTPS-only and it does work.
- Downloads are now multithreaded directly in the downloader.
- Requests and Curl-Impersonate use one singular Session for all downloads, keeping connections alive and cached so it doesn't have to close and reopen connections for every single download.
- Progress updates are now yielded back to the caller instead of drilling down a progress callable.
The browser to imitate can be set in the config:
For example,
```yaml
curl_impersonate:
browser: chrome110
```
It will default to using chrome110 if no value is set in the config.
A list of available Browsers are listed here: https://github.com/yifeikong/curl_cffi#sessions
