[feature] gRPC File Download API

[KCarretto]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Adds a new gRPC method for downloading files.

[Cictrone]

make sure imix has that cool new method, but otherwise lgtm

[hulto]

https://docs.realm.pub/dev-guide/tavern
Should we update the docs around the download API?

[hulto]

One question otherwise LGTM

[codecov]

Codecov Report

Attention: 33 lines in your changes are missing coverage. Please review.

Comparison is base (3576b5c) 71.46% compared to head (7ff3d25) 71.30%.

Files	Patch %	Lines
implants/lib/c2/src/c2.rs	0.00%	24 Missing ⚠️
tavern/internal/c2/api_download_file.go	77.50%	6 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #447      +/-   ##
==========================================
- Coverage   71.46%   71.30%   -0.17%     
==========================================
  Files         115      116       +1     
  Lines        8835     8900      +65     
==========================================
+ Hits         6314     6346      +32     
- Misses       2412     2442      +30     
- Partials      109      112       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.