Added support for KEYRING storage of the password of ROSDEP

[progtologist]

I did some initial testing with both python2 and python3 but I cannot vouch 100% about the compatibility of everything. If we can get a publicly accessible authorized rosdep server I can write proper unit tests for everything.

I also took a shot at documenting this!

[codecov-commenter]

Codecov Report

Base: 74.89% // Head: 74.83% // Decreases project coverage by -0.06% ⚠️

Coverage data is based on head (b119278) compared to base (1153836).
Patch coverage: 61.11% of modified lines in pull request are covered.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           cottsay/auth_urls     #895      +/-   ##
=====================================================
- Coverage              74.89%   74.83%   -0.07%     
=====================================================
  Files                     44       44              
  Lines                   3374     3389      +15     
=====================================================
+ Hits                    2527     2536       +9     
- Misses                   847      853       +6     

Impacted Files	Coverage Δ
src/rosdep2/url_utils.py	82.97% <61.11%> (-10.78%)	⬇️

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