feat: replace downloader dependency with reqwest

[TomAFrench]

This PR replaces downloader with reqwest

This results in the progress bar being displayed as below:

// Downloader
Downloading the Ignite SRS (340MB)

[00:00:17] ######################################## 322560348/322560348 
Downloaded the SRS successfully!
Time Elapsed: 17 seconds

// Reqwest
Downloading the Ignite SRS (340MB)

[00:00:17] ######################################## 307.62 MiB/307.62 MiB Downloaded the SRS successfully!
SRS is located at: "/home/tom/.nargo/backends/acvm-backend-barretenberg/transcript00.dat"

The "Downloaded the SRS successfully!" message only appears on completion of the download. I've removed the "Time Elapsed" line as this is displayed in the progress bar.

Resolves #47

[TomAFrench]

This PR just aims to be a seamless replacement for the downloader crate. We can revisit with avoiding panics in future.

[phated]

I haven't tested this locally, but let's get it landed to avoid additional churn while we do the refactors.