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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.73%. Comparing base (1cdac66) to head (f075197).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #941      +/-   ##
==========================================
+ Coverage   80.66%   80.73%   +0.06%     
==========================================
  Files          24       24              
  Lines        1443     1443              
  Branches      206      206              
==========================================
+ Hits         1164     1165       +1     
  Misses        187      187              
+ Partials       92       91       -1     

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[dwo]

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