Refactor basic auth middleware to support multiple auth headers

[aldas]

This is taken from v5 branch + improved tests

reasoning for it:
multiple auth headers is something that can happen in environments like corporate test environments that are secured by application proxy servers where front facing proxy is configured to require own basic auth value + checks it and your application also requires basic auth headers from clients. As Go standard library stores headers in map and keys are retrieved in random order the middleware may need to check multiple headers to match correct one.

[codecov]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.09%. Comparing base (447c92d) to head (4ab78e4).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
middleware/basic_auth.go	94.87%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2695      +/-   ##
==========================================
+ Coverage   93.03%   93.09%   +0.05%     
==========================================
  Files          41       40       -1     
  Lines        4707     4762      +55     
==========================================
+ Hits         4379     4433      +54     
- Misses        236      238       +2     
+ Partials       92       91       -1     

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