Allow specifying the username

[danielmatz]

Libcurl's SSH key-based and agent-based authentication methods don't work unless the username is set, either by including it in the URL or by setting CURLOPT_USERNAME. This PR adds a username keyword to the download and request functions to allow the user to optionally specify a username.

Related to this discourse discussion: https://discourse.julialang.org/t/privately-hosting-binarybuilder-products-with-authentication/88409

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.81%. Comparing base (3ed0f08) to head (ae93f7d).
⚠️ Report is 24 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Curl/Easy.jl	0.00%	2 Missing ⚠️
src/Downloads.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #225      +/-   ##
==========================================
- Coverage   90.26%   89.81%   -0.46%     
==========================================
  Files           5        5              
  Lines         575      579       +4     
==========================================
+ Hits          519      520       +1     
- Misses         56       59       +3     

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[danielmatz]

It seems the single CI failure is caused by a lack of test coverage of this diff. I wasn't really sure how to test this, though. Does anyone have tips on that?