Use mixed as push middleware definition format

[vjik]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Tests pass?	✔️

[codecov]

Codecov Report

❌ Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.28%. Comparing base (5e9818c) to head (7cace45).

Files with missing lines	Patch %	Lines
src/Debug/QueueDecorator.php	0.00%	2 Missing ⚠️
src/Middleware/Push/PushMiddlewareFactory.php	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #286      +/-   ##
============================================
- Coverage     98.39%   98.28%   -0.12%     
- Complexity      324      327       +3     
============================================
  Files            48       48              
  Lines           873      875       +2     
============================================
+ Hits            859      860       +1     
- Misses           14       15       +1     

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