fix: Use URL-safe base64 encoding for `X-Registry-Auth`

[dwo]

What do these changes do?

Changes utils.compose_auth_header to use base64.urlsafe_b64encode on the returned string. This aligns with how the Docker CLI encodes the same header.

I discovered this when an application using aiodocker was the only client not authenticating successfully against a private registry after a service user's password was rotated.

Previously raised by me in #761.

Are there changes in behavior for the user?

Will allow authentication to succeed against private registries when the X-Registry-Auth require url-safe substitutions as decribed here: https://docs.python.org/3/library/base64.html#base64.urlsafe_b64encode

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
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[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 81.57%. Comparing base (6ae6c59) to head (2d0a1a2). :warning: Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #941      +/-   ##
==========================================
+ Coverage   81.50%   81.57%   +0.07%     
==========================================
  Files          24       24              
  Lines        1422     1422              
  Branches      190      190              
==========================================
+ Hits         1159     1160       +1     
  Misses        186      186              
+ Partials       77       76       -1     

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[dwo]

@asvetlov just bumping this in case you still want to review this fix.